NOx Trap Efficiency Below Threshold (Bank 1)

Possible causes

• CommonTrap poisoned by sulphur, which builds up over tens of thousands of miles
• CommonMostly short journeys, so the trap never reaches operating temperature
• CommonFaulty NOx sensor reporting poor conversion that is not real
• CommonExhaust leak before or between the sensors
• PossibleEngine running outside its normal mixture range, so regeneration cannot complete
• PossibleTrap genuinely worn out

What you might notice

• Check engine light on
• Failed emissions test
• Worse fuel economy
• Reduced power in some cases
• Often no symptom the driver notices

What to check, in order

1. Read all codes first. A NOx sensor code alongside this one usually names the real fault, and sensors are far cheaper than traps.
2. Check for exhaust leaks between the engine and the sensors. Air drawn in through a leak makes a working trap measure badly.
3. Consider how the car is used. A vehicle doing only short cold journeys cannot regenerate properly, and a long run at sustained speed sometimes clears the condition.
4. Do not replace the trap first. It is one of the most expensive parts in the exhaust and the sensors around it fail far more often.
5. Check whether the engine has other faults affecting mixture. A trap cannot regenerate correctly on an engine that is running wrong.