What does it mean?
This code is set when the fourth oxygen sensor on bank 1 still produces a signal, but that signal changes too slowly for the module to accept it as accurate. During a dedicated response test the module forces the mixture to swing and measures how long the sensor needs to travel between its rich and lean thresholds; if that transition time exceeds the calibrated limit, the sensor is called lazy. A slow rear sensor is usually a chemical or thermal problem rather than a wiring one: the sensing ceramic is coated, aged or running below its light-off temperature. Because catalyst efficiency is judged from the shape of this signal, the module cannot trust the catalyst monitor while the sensor responds slowly. The MIL comes on after the test fails on repeated drive cycles.
Symptoms
- MIL on with the catalyst monitor stuck incomplete
- Slightly higher fuel consumption and marginally worse emissions numbers at inspection
- Sluggish, lazy-looking trace when the sensor is graphed next to the upstream sensor
- Often no driveability complaint at all, the light being the only sign
- Occasional hesitation on light throttle if the module leans on this sensor for trim
Likely causes
- Normal ageing of the sensing element after high mileage, which is the most common finding
- Contamination of the ceramic by oil ash, coolant, silicone from sealants or fuel additives
- Weak or failing heater circuit that leaves the sensor below its operating temperature, especially at idle and low load
- Soot loading of the element after long idling, repeated short trips or a period of rich running
- Small exhaust leak ahead of the sensor that dilutes the sample and softens every transition
- High resistance in the signal or return wiring that rounds off the edges of the waveform
- A poor-quality replacement sensor that never had the response speed of the original
How to fix it
- Graph the sensor with the engine fully warm and force a rich and lean sweep; compare its transition time with the upstream sensor on the same graph
- Confirm the sensor actually reaches operating temperature: check heater fuse, heater feed voltage and heater resistance, and look at how quickly the signal comes alive from a cold start
- Inspect the whole exhaust ahead of the sensor for small leaks, especially at flanges, flex sections and the sensor bung threads
- Measure the signal and return circuits for added resistance and repair any corroded splices or connector terminals
- Look for an engine condition that could be poisoning the element, such as oil consumption, a coolant leak or the recent use of a non-sensor-safe sealant
- Replace the sensor once heating, wiring and exhaust integrity have been proven, then rerun the monitor to confirm the response time is now within limits