Code
P1157
GMC
P — Powertrain
HO2S Bank 2 Sensor 2 Lean System Or Low Voltage
Views:
UK: 27
EN: 45
RU: 35
AI status
Completed
Completed
100%
Causes
- Failed or degraded Bank 2 Sensor 2 (downstream HO2S)
- Open, shorted, corroded or damaged wiring/connectors to the sensor
- Exhaust leak upstream of the sensor or near Bank 2
- Low fuel pressure or fuel delivery problem causing a lean condition
- Intake air leak (vacuum leak) on Bank 2
- Mass airflow (MAF) or MAP sensor errors causing incorrect fuel trim
Symptoms
- Check Engine Light (MIL) illuminated
- Possible reduced fuel economy or drivability issues
- Failed emissions test (elevated tailpipe emissions)
- Possible rough idle or hesitation under some conditions (if lean condition is present)
- Stored freeze frame data showing conditions when the code set
What to check
- Read and record freeze frame and live data with a capable scan tool (fuel trims, O2 sensor voltages, MAF/MAP, coolant temp, engine load)
- Visually inspect sensor, connector and wiring for damage, corrosion, or disconnection
- Backprobe sensor signal and heater leads with engine running to observe voltage and heater control
- Compare upstream (Bank 2 Sensor 1) and downstream (Bank 2 Sensor 2) O2 voltages
- Check for exhaust leaks on Bank 2 (manifold, gaskets, pipes) and repair if found
- Measure fuel pressure at the rail and inspect injectors for proper operation
Signal parameters
- Downstream HO2S (Sensor 2) typically shows a relatively steady voltage near ~0.45 V with small fluctuations when catalytic converter is operating; it should not switch rapidly like upstream sensor
- A persistent low voltage (< ~0.1–0.2 V) at the downstream sensor indicates a lean signal or low sensor output
- Upstream HO2S (Sensor 1) normally switches between ~0.1–0.9 V under closed‑loop operation; large differences between upstream and downstream suggest sensor or catalyst issues
- Heater circuit: expected continuity across heater element (varies by sensor design). Open heater or no 12V/PCM pulsed feed can prevent proper operation
Diagnostic algorithm
- Retrieve freeze frame, pending and related codes. Note conditions (RPM, load, temp) when code set.
- Perform a visual inspection of Bank 2 Sensor 2 wiring and connector; repair any damage or corrosion.
- With a scan tool, monitor Bank 2 Sensor 1 and Sensor 2 voltages in real time. Confirm whether Sensor 2 is low/stuck while Sensor 1 is switching normally.
- Backprobe the sensor: verify signal voltage and check for correct reference/ground and heater supply (with ignition on and engine running where applicable).
- Check heater circuit resistance and continuity to PCM. Replace sensor if heater is open or heater circuit fails.
- Check for exhaust leaks upstream of the downstream sensor and repair any leaks, then retest.
- Inspect fuel system: measure fuel pressure and inspect injectors on Bank 2 for proper operation. Address low fuel pressure or injector faults.
- Check for intake vacuum leaks (smoke test) and clean/test MAF and MAP sensors; correct any faulty air metering.
- If wiring, power and fuel systems test good and downstream sensor still reads low, consider replacing Bank 2 Sensor 2. After replacement, clear codes and road test to confirm resolution.
- If code returns after sensor replacement, inspect catalytic converter efficiency on bank 2 and PCM operation; further module or catalyst evaluation may be required.
Likely causes
- Aging or contaminated downstream O2 sensor producing low/erratic voltage
- Wiring harness damage or connector corrosion at sensor
- Exhaust leak on bank 2 allowing extra oxygen at the sensor
- Fuel pressure low or injector issue on bank 2 producing real lean condition
- Heater circuit failure in the sensor (sensor not up to operating temperature)
Fault status
Status
Bank 2 Sensor 2 (downstream HO2S) is reporting a low/lean voltage or heater/signal fault. This can be caused by a failed sensor, wiring/connector problems, exhaust or intake leaks, fuel delivery issues, or catalytic converter faults. Further testing is required to determine the root cause.
Repair difficulty: Medium
Diagnostic time: 1–3 hours
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