Code
P0151
HUMMER
P — Powertrain
Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 2 Sensor 1
AI status
Completed
Completed
100%
Causes
- Faulty HO2S (upstream) on Bank 2
- Open or shorted wiring or connector to the sensor (signal or heater)
- Exhaust leak upstream of the sensor
- Fuel delivery problems causing excessively lean mixture
- Sensor contaminated by coolant, oil, silicone or leaded fuel
- ECM or ground fault
Symptoms
- Check Engine light (MIL) illuminated
- Poor fuel economy
- Rough idle or hesitation if fuel trims are driven out of range
- Possible failed emissions test
- Elevated or unstable long-term fuel trim for Bank 2
What to check
- Read freeze frame and live data with scan tool; observe Bank 2 Sensor 1 voltage and short/long term fuel trims
- Compare Bank 1 Sensor 1 behavior to Bank 2 Sensor 1
- Inspect sensor wiring and connector for damage, corrosion, or disconnection
- Check for exhaust leaks upstream of the sensor (manifold/gaskets)
- Measure sensor heater circuit voltage and resistance
- Check vehicle fuel pressure and injector operation for lean condition
Signal parameters
- Typical narrowband O2 voltage: ~0.1 V = lean, ~0.9 V = rich; P0151 indicates the signal is stuck low (near 0–0.1 V)
- When closed-loop, upstream O2 should switch between low (~0.1–0.3 V) and high (~0.7–0.9 V) several times per second
- Heater circuit: supply usually ~12 V with ground controlled or fixed; heater resistance commonly ~3–20 Ω (manufacturer-specific)
- Expected switching frequency in normal closed-loop: roughly 0.5–2 Hz depending on engine/load
- If heater is dead, sensor warm-up time increases and switching may not occur until hot
Diagnostic algorithm
- Retrieve freeze frame and live PID data. Confirm Bank 2 Sensor 1 voltage is low and not switching while engine is at normal operating temperature.
- Visually inspect sensor and harness for heat damage, chafing, corrosion, or disconnected connector. Repair any obvious damage.
- Backprobe the signal wire at the sensor connector with a voltmeter/scan tool while engine running: verify actual voltage. If signal reads normal at connector but ECM sees low, suspect harness/ECM connector.
- Check continuity and shorts: with ignition off, measure resistance between signal wire and ground (should not be shorted). Check for short to power where applicable.
- Test heater circuit: measure voltage at heater pins with key on (power) and engine running; measure heater resistance with sensor removed. Replace sensor if heater open or out of spec.
- Inspect for exhaust leaks upstream of the sensor; repair leaks and re-test.
- Check fuel system: fuel pressure, injector operation and Lambda/fuel trims. Repair any causes of a lean condition (vacuum leaks, low fuel pressure, injector issues).
- If wiring and system checks good but problem persists, replace the Bank 2 Sensor 1 with a correct OEM-spec sensor and clear codes. Re-test drive cycle to confirm code does not return.
- If code returns after sensor replacement and wiring checks are good, consider ECM or ground integrity tests and consult manufacturer technical service information.
Likely causes
- Damaged sensor harness with internal short to ground causing low signal
- Failed oxygen sensor (internal heater or sensing element)
- Disconnected or corroded sensor connector pin(s)
- Exhaust leak before sensor pulling in ambient air
- Restricted fuel injector(s) or low fuel pressure causing lean condition
- Blown fuse or open circuit in sensor heater power/ground (affecting sensor response)
Fault status
Status
Heated Oxygen Sensor (HO2S) Circuit Low Voltage — Bank 2 Sensor 1: sensor signal is abnormally low or stuck low indicating lean reading or electrical fault.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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