Code
P0143
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 3
AI status
Completed
Completed
100%
Causes
- Failed or degrading HO2S (sensor element)
- Open or shorted sensor signal wire (to ground or low potential)
- Corroded/loose sensor connector or poor pin contact
- Faulty sensor heater circuit or blown fuse/relay
- Exhaust leak near the sensor causing abnormal readings
- Contaminated sensor (oil, coolant, silicone, or fuel additives)
Symptoms
- Check Engine MIL illuminated
- Possible reduced fuel economy or rough running in some cases
- Failed emissions test (catalyst monitoring may fail)
- Possible lack of closed-loop fuel control from downstream monitoring
- Diagnostic trouble code stored (P0143) and freeze frame data
What to check
- Retrieve freeze frame and live data with a scan tool; confirm P0143 and related codes
- Visually inspect sensor, connector, and wiring for damage, corrosion, heat damage, or disconnection
- Check for exhaust leaks near sensor mounting points
- Backprobe sensor signal and heater pins to observe voltage and switching behavior
- Measure heater circuit resistance and compare to specification
- Check relevant fuses/relays and PCM ground/connector integrity
Signal parameters
- Sensor signal voltage (HO2S narrowband): ~0.0–1.0 V; typical lean ~0.1–0.3 V, rich ~0.7–0.9 V
- Downstream sensor behavior: less switching than upstream; may sit near ~0.4–0.6 V if catalyst functioning
- Low-voltage fault: sensor signal below PCM threshold or no switching activity
- Heater circuit supply: ~11–14 V when commanded (vehicle battery voltage)
- Typical heater resistance (varies by sensor): approximate range 2–14 Ω (refer to vehicle spec)
- Heater current draw: commonly ~0.5–3 A depending on design
Diagnostic algorithm
- Confirm code and review freeze-frame/live data for Bank 1 Sensor 3 and upstream sensors.
- Visually inspect connector and wiring harness for damage, chafing, corrosion, or water intrusion.
- Check for exhaust leaks upstream of the downstream sensor and repair if present.
- With ignition on (engine off), measure voltage at heater supply pin; expect battery voltage when PCM commands heater; if no voltage, check fuse/relay and wiring to PCM.
- Measure heater resistance at the sensor connector; if open or out of range, replace sensor.
- Backprobe the sensor signal with engine running; confirm signal voltage and compare to expected downstream behavior. Look for continuously low voltage (stuck low) or no signal.
- Check continuity and for shorts between signal, heater, and ground circuits to chassis and battery using an ohmmeter.
- If wiring and heater are good, substitute a known good sensor or install a new sensor, then clear codes and road test to verify.
- If new sensor still faults and wiring checks good, suspect PCM driver fault — verify PCM power/grounds and consider module testing or replacement per manufacturer procedures.
Likely causes
- Wiring short to ground on the sensor signal/ground conductor
- Damaged connector pins or corrosion at sensor harness
- Aged downstream oxygen sensor that no longer produces expected voltage
- Heater circuit open or high resistance preventing proper sensor operation
- Exhaust leak between upstream and downstream sensors affecting readings
Fault status
Status
Heated oxygen sensor (Bank 1, Sensor 3) voltage is lower than expected or not present. The engine control module detected an abnormal low-voltage condition on the downstream O2 sensor circuit, which may indicate a faulty sensor, wiring/connector problem, heater circuit issue, or related exhaust/system fault.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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