Code
P1155
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Lean Mean Bank 2 Sensor 1
AI status
Completed
Completed
100%
Causes
- Faulty Bank 2 Sensor 1 (HO2S) -- slow or biased output
- Intake vacuum leak on Bank 2 (unmetered air)
- Low fuel pressure or weak fuel pump/regulator
- Mass Air Flow (MAF) sensor or intake air sensor fault/contamination
- Exhaust leak upstream of Bank 2 sensor
- Wiring problem or poor connector/ground to HO2S or heater circuit
Symptoms
- Check Engine Light (MIL) illuminated
- Lean fuel trims (positive long-term and short-term fuel trim on Bank 2)
- Poor idle quality or roughness
- Hesitation or reduced engine power under acceleration
- Possible reduced fuel economy or increased NOx emissions
What to check
- Read freeze frame and live data with a capable scan tool (monitor Bank 2 O2 voltage, short-term and long-term fuel trims, engine load, MAF, fuel trims)
- Check for related codes (heater circuit, fuel trim, MAF)
- Visually inspect intake and vacuum lines for cracks, disconnected hoses, or improper connections on Bank 2
- Inspect exhaust upstream of sensor for leaks or cracks
- Perform fuel pressure test at fuel rail to verify proper pressure and regulator operation
- Check MAF sensor operation and cleanliness; compare airflow readings to expected values
Signal parameters
- HO2S (narrowband) voltage: typically ~0.1 V = lean, ~0.9 V = rich; a healthy upstream sensor will switch between low and high roughly 1–2 Hz at idle in closed loop
- Bank 2 Sensor 1 expected behavior: active switching in closed loop; long-lasting low voltage (
- Short-term fuel trim (STFT) on Bank 2: significant positive values (+10% or more) indicate compensation for lean condition
- Heater circuit: verify battery voltage present on supply lead with key on; typical heater resistance often in single-digit to low-double ohm range (manufacturer-specific)
Diagnostic algorithm
- Connect a scan tool and record freeze-frame and live data (focus on Bank 2 O2 voltage, STFT/LTFT, MAF, fuel pressure). Note conditions when code set.
- Confirm O2 sensor signal: with engine at operating temp in closed loop, observe Bank 2 Sensor 1 voltage — it should switch between ~0.1–0.9 V. If it stays low, proceed.
- Inspect for intake vacuum/exhaust leaks: perform visual inspection and a smoke test of intake and vacuum plumbing; inspect exhaust manifold and pipe for leaks upstream of the sensor.
- Check fuel supply: measure fuel pressure at the rail and compare to spec for the vehicle. Repair fuel pump/regulator/filter if pressure is low.
- Evaluate MAF and intake air measurement: check for contamination, perform cleaning if required, and verify airflow values at idle and at higher rpms.
- Inspect sensor wiring and connector: check for corrosion, broken wires, short to power/ground, and good ground continuity. Wiggle wiring while watching live sensor signal for intermittent faults.
- Check HO2S heater circuit: measure resistance and verify 12 V supply on heater fuse/circuit when key on. Repair heater circuit if open.
- If wiring and vehicle systems check good, consider replacing the Bank 2 Sensor 1 HO2S with OE or equivalent sensor. After replacement, clear codes and verify proper switching and fuel trim correction.
- If problem persists after sensor replacement, perform targeted injector tests on Bank 2, verify compression and engine mechanical condition, and re-check for air leaks or wiring issues.
Likely causes
- Vacuum or intake leak on bank 2
- Failed or slow-response upstream oxygen sensor (HO2S Bank 2, Sensor 1)
- Low fuel pressure or clogged fuel filter
- MAF sensor contamination or incorrect intake air reading
- Damaged wiring or poor sensor ground
Fault status
Status
ECM detected an oxygen sensor (HO2S) lean signal on Bank 2 Sensor 1 and set DTC P1155. Malfunction Indicator Lamp (MIL) may be on. Further diagnosis required—do not assume sensor replacement without verification.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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