P042B

What it means

P042B is a generic code meaning the catalyst temperature sensor circuit for Bank 1, Sensor 2 (downstream of the catalytic converter) is reporting a value that is out of range or does not behave plausibly. The control unit compared the sensor signal with its modelled catalyst temperature or with expected changes in temperature and found a mismatch. The exact sensor position and how the strategy works depend on the model and engine.

Symptoms

• Engine warning light on
• Usually no noticeable change in driving
• Possible reduced power or limp mode on some models
• Other catalyst or oxygen sensor codes stored alongside
• Possible failed emissions test

Possible causes

• Faulty catalyst temperature sensor (drifted, slow or failed)likely
• Damaged, chafed, corroded or heat-affected wiring or connector to the sensorlikely
• Exhaust leak near the sensor affecting temperature readingspossible
• Degraded or overheated catalytic converter giving abnormal temperature behaviourpossible
• Engine misfire or rich or lean running causing abnormal catalyst temperaturespossible
• Fault in the control unit or its sensor supply or reference circuitrare

How to diagnose

1. Read all stored and pending codes and freeze frame data, and note any misfire, fuelling or oxygen sensor codes that need dealing with first
2. Visually inspect the sensor, its wiring and connector for heat damage, chafing, corrosion and water ingress
3. Check for exhaust leaks around the catalytic converter and sensor mounting
4. Using a scan tool, watch the live catalyst temperature sensor reading when cold and as the engine warms up, and check it changes plausibly
5. Check the sensor's resistance or signal against the manufacturer's specification for your model
6. Check the wiring for continuity, shorts to earth or supply, and check the reference voltage at the connector
7. Check fuel trims and misfire counters to rule out running problems that could overheat the catalyst
8. Assess catalytic converter condition, for example with a temperature comparison before and after it, following manufacturer procedure

How to fix

• Replace the catalyst temperature sensor if it is out of specification
• Repair or replace damaged wiring or connectors and clear any corrosion
• Repair exhaust leaks near the sensor
• Correct any misfire or fuelling fault, then replace the catalytic converter if it has been damaged
• Replace the control unit only when confirmed faulty, or apply a manufacturer calibration update if one is a known fix for the model

Can I keep driving?

Nearby codes

• P042AGenericCatalyst Temperature Sensor Circuit Bank 1 Sensor 2
• P042CGenericCatalyst Temperature Sensor Circuit Low Bank 1 Sensor 2