Code
P0138
DODGE
P — Powertrain
HO2S12 High Voltage
Views:
UK: 33
EN: 70
RU: 117
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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Code
P0138
Generic
P — Powertrain
O2 Sensor Circuit High Voltage Bank 1 Sensor 2
Views:
UK: 34
EN: 69
RU: 111
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
Similar codes
Repair manuals
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Code
P0138
GWM
P — Powertrain
- High voltage of oxygen sensor heating circuit (bank 1, sensor 2)
Views:
UK: 11
EN: 31
RU: 67
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
Similar codes
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Code
P0138
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 2
Views:
UK: 23
EN: 42
RU: 80
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
Similar codes
Repair manuals
Manual library for HUMMER
69
Browse 69 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
HUMMER
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Code
P0138
LAND ROVER
P — Powertrain
high voltage O2 sensor circuit (bank 1 sensor 2)
Views:
UK: 17
EN: 42
RU: 82
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
Similar codes
Repair manuals
Manual library for LAND ROVER
160
Browse 160 LAND ROVER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
LAND ROVER
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LAND ROVER: 2023
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Range Rover Velar
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LAND ROVER: 2022
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Range Rover
- Autobiography, 4.4L Eng VIN 7 · 4.4L Eng VIN 72022: Range Rover Autobiography
- Autobiography, 5.0L Eng VIN E · 5.0L Eng VIN E2022: Range Rover Autobiography
- 2022 Range Rover Base
- First Edition
- HSE Westminster, 3.0L Eng VIN U · 3.0L Eng VIN U2022: Range Rover HSE Westminster
- HSE Westminster, 5.0L Eng VIN E · 5.0L Eng VIN E2022: Range Rover HSE Westminster
- SE, 3.0L Eng VIN U · 3.0L Eng VIN U2022: Range Rover SE
- SE, 4.4L Eng VIN 7 · 4.4L Eng VIN 72022: Range Rover SE
- SV
- SVAutobiography
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- SVAutobiography Dynamic Blk.
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Range Rover Evoque
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Range Rover Sport
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Range Rover Velar
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LAND ROVER: 2021
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Discovery Sport
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Range Rover
- Autobiography, 2.0L Eng VIN Y · 2.0L Eng VIN Y2021: Range Rover Autobiography
- Autobiography Fifty Edition
- 2021 Range Rover Base
- HSE, 2.0L Eng VIN Y · 2.0L Eng VIN Y2021: Range Rover HSE
- HSE, 3.0L Eng VIN K · 3.0L Eng VIN K2021: Range Rover HSE
- HSE Westminster, 3.0L Eng VIN U · 3.0L Eng VIN U2021: Range Rover HSE Westminster
- HSE Westminster, 5.0L Eng VIN E · 5.0L Eng VIN E2021: Range Rover HSE Westminster
- SVAutobiography
- SVAutobiography Dynamic
- SVAutobiography Dynamic Blk.
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Range Rover Evoque
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Range Rover Sport
- Autobiography, 2.0L Eng VIN Y · 2.0L Eng VIN Y2021: Range Rover Sport Autobiography
- Autobiography, 5.0L Eng VIN E · 5.0L Eng VIN E2021: Range Rover Sport Autobiography
- Autobiography Dynamic, 2.0L Eng VIN Y · 2.0L Eng VIN Y2021: Range Rover Sport Autobiography Dynamic
- Autobiography Dynamic, 5.0L Eng VIN E · 5.0L Eng VIN E2021: Range Rover Sport Autobiography Dynamic
- HSE Dynamic
- HSE Silver Edition, 2.0L Eng VIN Y · 2.0L Eng VIN Y2021: Range Rover Sport HSE Silver Edition
- HSE Silver Edition, 3.0L Eng VIN K · 3.0L Eng VIN K2021: Range Rover Sport HSE Silver Edition
- HSE Silver Edition, 3.0L Eng VIN U · 3.0L Eng VIN U2021: Range Rover Sport HSE Silver Edition
- HST
- SE
- SVR
- SVR Carbon Edition
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Range Rover Velar
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LAND ROVER: 2020
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Discovery
- HSE, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Discovery HSE
- HSE, 3.0L Eng VIN V · 3.0L Eng VIN V2020: Discovery HSE
- HSE Luxury, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Discovery HSE Luxury
- HSE Luxury, 3.0L Eng VIN V · 3.0L Eng VIN V2020: Discovery HSE Luxury
- Landmark
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- SE, 3.0L Eng VIN V · 3.0L Eng VIN V2020: Discovery SE
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Discovery Sport
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Range Rover
- Autobiography
- Base, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Range Rover Base
- Base, 3.0L Eng VIN U · 3.0L Eng VIN U2020: Range Rover Base
- HSE, 2.0L Eng VIN Y · 2.0L Eng VIN Y2020: Range Rover HSE
- HSE, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Range Rover HSE
- HSE, 3.0L Eng VIN U · 3.0L Eng VIN U2020: Range Rover HSE
- HSE, 5.0L Eng VIN E · 5.0L Eng VIN E2020: Range Rover HSE
- SVAutobiography
- SVAutobiography Dynamic
-
Range Rover Evoque
-
Range Rover Sport
- Autobiography Dynamic, 2.0L Eng VIN Y · 2.0L Eng VIN Y2020: Range Rover Sport Autobiography Dynamic
- Autobiography Dynamic, 5.0L Eng VIN E · 5.0L Eng VIN E2020: Range Rover Sport Autobiography Dynamic
- HSE, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Range Rover Sport HSE
- HSE, 3.0L Eng VIN U · 3.0L Eng VIN U2020: Range Rover Sport HSE
- HSE, 5.0L Eng VIN E · 5.0L Eng VIN E2020: Range Rover Sport HSE
- HSE Dynamic
- HSE PHEV
- HST
- SE, 3.0L Eng VIN K · 3.0L Eng VIN K2020: Range Rover Sport SE
- SE, 3.0L Eng VIN U · 3.0L Eng VIN U2020: Range Rover Sport SE
- SVR
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Range Rover Velar
- R-Dynamic HSE
- R-Dynamic S, 2.0L Eng VIN X · 2.0L Eng VIN X2020: Range Rover Velar R-Dynamic S
- R-Dynamic S, 3.0L Eng VIN V · 3.0L Eng VIN V2020: Range Rover Velar R-Dynamic S
- S, 2.0L Eng VIN X · 2.0L Eng VIN X2020: Range Rover Velar S
- S, 3.0L Eng VIN V · 3.0L Eng VIN V2020: Range Rover Velar S
- SVAutobiography Dyn.
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LAND ROVER: 2019
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Code
P0138
MITSUBISHI
P — Powertrain
Oxygen sensor(rear) high
Views:
UK: 19
EN: 43
RU: 80
AI status
Completed
Completed
100%
Causes
- Faulty downstream O2 sensor (Bank 1, Sensor 2)
- Short to battery voltage in sensor signal circuit
- Rich air/fuel condition (high fuel pressure, leaking injector, stuck open fuel pressure regulator)
- Contaminated sensor (oil, coolant, silicone)
- PCM internal fault or poor ground
- Wiring or connector corrosion, damage or poor pin contact
Symptoms
- Check Engine Light (MIL) illuminated
- Failing or poor emissions test (high HC/CO)
- Reduced fuel economy (if system running rich)
- Possible rough idle or black exhaust smoke under heavy load
- Normally no drivability symptoms if issue is electrical only
What to check
- Read and record freeze frame and pending/multiple codes
- Scan live data — monitor B1S1 and B1S2 O2 voltages and short/long term fuel trims (STFT/LTFT)
- Visually inspect sensor wiring and connector for damage, corrosion, pin pushed out, or melted insulation
- Check for exhaust leaks upstream of the sensor
- Measure sensor signal voltage with scan tool or multimeter (backprobe)
- Check heater circuit resistance and power/ground to the heater
Signal parameters
- O2 sensor voltage range: ~0.0–1.0 V (narrowband)
- Upstream (B1S1) should switch rapidly ~0–1 V; downstream (B1S2) should be steadier ~0.45 V at closed loop
- High-voltage trip typically >0.8–0.9 V (manufacturer threshold varies)
- Heater resistance (varies by sensor, typically 2–20 Ω) — compare to spec
- Long/short term fuel trims: positive trims indicate lean, negative trims indicate rich
Diagnostic algorithm
- Verify code and record freeze frame. Clear codes and recheck to confirm repeatability.
- Inspect sensor and harness: look for chafing, heat damage, corrosion, or disconnected pins at B1S2 connector.
- With a scan tool, monitor live B1S1 and B1S2 voltages and fuel trims. Note if B1S2 is stuck high while B1S1 behaves normally.
- Backprobe B1S2 signal wire with voltmeter/scan tool. If voltage is above ~0.8–0.9 V steady, suspect sensor or short to voltage.
- Unplug B1S2 sensor and observe signal: if signal drops to 0 V or open-circuit behavior, the harness may be okay; if it remains high, check for short to battery in wiring or PCM.
- Check for short to power: with ignition on, measure voltage between signal wire and ground; remove connectors as needed to isolate harness from PCM. Repair shorted wiring if found.
- Check heater circuit: measure resistance and verify 12V power and ground control from PCM (if heater circuit fault present, also examine P0135/P0141 codes).
- If wiring and PCM outputs are good and no fuel system cause is found, replace downstream O2 sensor. Use OEM or quality equivalent.
- After repair, clear stored codes and perform a road test to ensure proper operation and no reoccurrence.
Likely causes
- Sensor signal circuit shorted to 12V or battery feed
- Failed HO2S (sensor electronics stuck high)
- Excessive fuel delivery (fuel pressure regulator or leaking injector) causing a genuinely rich condition
- Exhaust contamination coating sensor
- Connector/ground problem
Fault status
Status
HO2S12 High Voltage — Bank 1 Sensor 2 signal above expected threshold (O2 sensor circuit high).
Repair difficulty: Medium
Diagnostic time: 1-3 hours
Similar codes
Repair manuals
Manual library for MITSUBISHI
203
Browse 203 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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