What does it mean?
The control module has seen the bank 1 sensor 4 signal stay above its highest plausible value for longer than the calibration allows. On a switching sensor anything held near or above one volt cannot be produced by normal exhaust chemistry at the tailpipe end of the system, so the module treats it as a high circuit fault. Two very different situations produce it: voltage pushed into the signal wire from a nearby power feed, or a lost signal return that lets the sensor output float upward. A genuinely very rich exhaust reaching the rear sensor can also drive the reading high, which is why fuel trims and upstream sensor data must be reviewed before touching the wiring. The code is stored with a freeze frame and the catalyst monitor is disabled until it is repaired.
Symptoms
- MIL on, often together with rich-condition or fuel trim codes when the mixture is really at fault
- Monitors do not complete and the vehicle fails an emissions inspection
- Black smoke, a strong exhaust smell or sooty tailpipe deposits when the exhaust really is rich
- Increased fuel consumption and, in bad cases, a rough idle
- Repeated converter overheating complaints if a genuinely rich mixture has been present for a long time
Likely causes
- Signal wire shorted to a voltage source, typically the sensor heater feed inside a damaged connector or a melted section of loom
- Open or high-resistance signal-return circuit letting the output float toward the reference supply
- Coolant or oil contamination of the sensing element from an internal engine leak, which biases the output high
- Internally shorted sensor after heat damage
- Genuinely rich exhaust reaching the rear of the system because of a leaking injector, excessive fuel pressure or a failed upstream sensor
- Moisture inside the connector creating a leakage path between the heater and signal pins
- Wrong sensor fitted, for example a wideband unit in a position calibrated for a switching sensor
How to fix it
- Compare the sensor 4 voltage with the upstream sensor and with short and long term fuel trims; if the whole system reads rich, chase the mixture, not the wiring
- Unplug the sensor with the key on; if the module still reads a high voltage, the harness is being fed from somewhere else
- Check the signal wire for a short to the heater feed or to battery voltage, and inspect the connector for melted or bridged terminals
- Confirm the signal return has continuity to the module and no offset against ground
- Look for coolant or oil consumption that could poison the element, including a leaking gasket or worn valve seals
- If the mixture is genuinely rich, test injector leakage and fuel pressure before replacing any sensor
- Fit the correct sensor last, clear the code and confirm normal switching on a road test