What does it mean?
This code means the voltage reported by the fourth oxygen sensor on bank 1 stayed below the lowest value the control module considers electrically possible for a calibrated period. A switching-type sensor in this position normally sits somewhere between roughly 0.1 and 0.9 volts and moves as the exhaust oxygen content changes, so a reading pinned near zero is treated as an electrical failure rather than a very lean mixture.
The module makes this call after checking that the engine was in a stable, closed-loop condition and that the sensor had enough time to heat up. Because a permanently low reading can also be produced by a genuine lean condition or by air entering a leaking exhaust joint ahead of the sensor, the diagnosis has to separate a wiring problem from an exhaust problem. Until the fault is cleared the catalyst monitor stays incomplete and the MIL is commanded on.
Symptoms
- MIL on and a stored freeze frame taken at a steady load
- Readiness monitors will not set, so the vehicle fails an emissions test
- Fuel trims may drift on vehicles that use this sensor for trim, showing up as slightly worse economy
- Occasional light hesitation or uneven idle if the module changes its fuel strategy
- A smell of unburned fuel or a soft exhaust leak noise from under the floor when a leaking joint is the real cause
Likely causes
- Signal wire shorted to ground, most often where the loom passes close to the exhaust or a suspension component
- Water or salt corrosion inside the sensor connector bridging the signal to the shield or return
- Open signal-return circuit, which lets the signal collapse toward ground
- Sensor element contaminated by coolant, oil ash or fuel additives and no longer able to generate voltage
- Genuine lean exhaust at that measuring point caused by a leaking joint, cracked pipe or failed weld ahead of the sensor
- Heater circuit fault leaving the sensor too cold to produce a normal output
- Incorrect or poor-quality replacement sensor with the wrong output characteristic
How to fix it
- Read the live sensor voltage first; a reading locked near 0 volts with the engine warm and the fuel trims normal points at the circuit, while a low reading that moves with throttle points at a real lean condition
- Unplug the sensor and check with the key on whether the module reads its own bias voltage; if the signal comes up to bias, the sensor or its harness is pulling it down
- Check the signal wire to ground and the shield with the connector open, then wiggle the loom while watching the meter
- Verify the return circuit and the module reference are within a few millivolts of each other
- Inspect every exhaust joint, flex pipe and weld ahead of the sensor for leaks with the engine cold and running
- Confirm the heater fuse and heater feed, then measure heater resistance
- Replace the sensor only after the wiring and the exhaust are proven sound, then verify switching activity on a road test