What does it mean?
After a cold start the module runs in open loop, using preset tables, until the oxygen or air fuel ratio sensor is hot enough to produce a usable signal. Once the sensor is ready the module switches to closed loop and begins trimming fuel from that feedback. This code is stored when the switch takes longer than the calibrated time under conditions where the loop should already have closed. The cause is either a sensor and heater that warm too slowly, a signal the module will not trust, or engine conditions such as a stuck open thermostat that keep the enabling criteria out of reach. Running open loop for too long wastes fuel and lets emissions rise well above normal.
Symptoms
- MIL on, typically after repeated short cold journeys
- Noticeably higher fuel consumption, especially in cold weather
- Rough idle, hesitation and a fuel smell during the first minutes after start
- Weak cabin heat and a temperature gauge that rises slowly
- Increased emissions and a failed emissions test
- Readiness monitors for the oxygen sensor and catalyst that will not set
Likely causes
- Weak or open oxygen sensor heater circuit, including a blown fuse or a corroded relay feed
- Aged, contaminated or lazy oxygen or air fuel ratio sensor that lights off slowly
- Exhaust leak upstream of the sensor drawing in outside air and preventing a stable signal
- Thermostat stuck open, so coolant temperature never reaches the closed loop enabling threshold
- Coolant temperature sensor reading low and blocking the transition
- High resistance in the sensor signal or ground circuits
- A very rich or very lean cold start condition caused by a fuel or air metering fault
How to fix it
- Record a cold start with a scan tool and note how long the sensor takes to start switching and when the module reports closed loop
- Check the heater circuit fuse, supply voltage and heater resistance at the sensor connector, and measure heater current if the equipment allows
- Watch coolant temperature during warm up to confirm the thermostat closes and the engine reaches normal temperature
- Verify the coolant temperature sensor against the real engine temperature
- Inspect the exhaust from the manifold to the sensor for leaks, using a smoke test if necessary
- Check fuel trims and fuel pressure once closed loop is reached, to rule out a mixture fault behind the slow entry
- Replace the sensor or thermostat as proven, then repeat the cold start recording to confirm faster closed loop entry