Code
P1136
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Rich Mean Bank 1 Sensor 1
AI status
Completed
Completed
100%
Causes
- Faulty/contaminated HO2S (Bank 1, Sensor 1)
- Damaged wiring or poor connector/ground to the sensor (open/short/high resistance)
- Fuel system overfueling (high fuel pressure, leaking or stuck-open injector(s))
- Incorrect MAF/MAP/Manifold pressure/airflow input (causing overfuelling)
- PCM or sensor heater circuit fault
- Intake or vacuum leak causing sensor cross-contamination (less common)
Symptoms
- Check Engine Light (MIL) on
- Poor fuel economy (running rich)
- Black exhaust smoke or strong fuel smell
- Rough idle or hesitation
- Failed emissions test (high HC/CO)
- Possible drivability complaints under some conditions
What to check
- Retrieve codes and freeze-frame data; note engine temp, load and conditions when code set
- Inspect sensor connector for corrosion, loose terminals, or water intrusion
- Backprobe O2 sensor signal (VS) and heater power/ground while running
- Confirm O2 sensor switching behavior and average voltage with a scan tool or oscilloscope
- Check short-term and long-term fuel trims (STFT/LTFT) for Bank 1
- Measure fuel rail pressure against spec and inspect for leaking injectors
Signal parameters
- Typical zirconia narrowband HO2S voltage range: ~0.1 V (lean) to ~0.9 V (rich)
- Normal closed-loop switching frequency: roughly 1–2 Hz under steady conditions
- Rich mean condition: sensor average voltage biased high (commonly > ~0.6 V over time or time-in-rich threshold set by OEM)
- Heater circuit: resistance commonly a few ohms (varies by sensor); heater supply should have battery voltage when key on
- Expected behavior: rapid oscillation about stoichiometry with fuel trims near 0% in normal operation
Diagnostic algorithm
- Confirm P1136 with a scan tool and review freeze-frame. Note engine temp, load, and operating conditions.
- Visually inspect HO2S connector and wiring for damage, pin corrosion, or loose terminals. Repair as needed.
- With engine at operating temp, monitor O2 sensor voltage, STFT/LTFT, and switching rate. Look for a steady high voltage (no switching) or an average biased rich.
- Check heater circuit: with key ON measure heater supply and ground; measure heater resistance at sensor and compare to spec.
- Check fuel pressure (static and running) and balance/inspect injectors for leakage or sticking. Repair any overfuel conditions.
- Test MAF/MAP and intake air system for incorrect readings or leaks (clean/replace MAF if contaminated).
- Perform wiring continuity and backprobe tests between sensor and PCM; check for shorts to voltage/ground or high resistance.
- If wiring and fuel/air systems are good, replace the upstream HO2S and clear codes. Re-test to confirm code does not return.
- If code returns after sensor replacement, pursue PCM/ground and module-level diagnostics or consult manufacturer-specific guidance.
Likely causes
- Failed or contaminated upstream oxygen sensor
- Corroded connector or broken/shorted wiring to HO2S
- High fuel pressure or leaking injector causing actual rich condition
- Faulty MAF or MAP giving incorrect air mass data
- Poor sensor ground or heater circuit failure
Fault status
Status
PCM has detected a sustained rich average signal from the heated oxygen sensor located at Bank 1 Sensor 1. This indicates the sensor or the fuel/air system may be causing or reporting a rich running condition.
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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