Emissions Light On? Troubleshooting Guide

Hello,

'For some time now, the emissions control light has occasionally been illuminating in my A4 BDG, with 35,000 km on the odometer. It's unclear under what driving conditions it occurs more frequently. Before the light comes on, there's a brief jolt and a momentary loss of power.'
The lamp stays on for a longer period, even after restarting the engine. After several hours of inactivity (overnight), it turns off. Unfortunately, this error has not occurred during appointments at the workshop. Reading the error memory by the workshop did not yield any results.

My question is: which values are checked by the exhaust emission control system, and can the cause of the error be narrowed down based solely on the illumination of the warning light?

Thank you in advance.
Best regards, Jean.

Let's try searching for that here as well.
Your engine control unit (ECU) monitors the mass airflow (MAF) value and can therefore determine the EGR rate. If the MAF value increases too much due to a stuck EGR valve, it can cause errors. Therefore, the problem may be a stuck EGR valve, and the error code should be checked.

Possible cause: EGR valve that is clogged or otherwise contaminated.

I'm assuming you're referring to the MIL (Mad Indication Lamp) lamp?

It also illuminates in case of sporadic errors and disappears again if the error does not occur for a specified period.
Actually, every event that causes the MIL (Malfunction Indicator Lamp) to illuminate is saved. However, when the lamp turns off due to a sporadic fault, the error entry should remain. At least, that is what the OBD (On-Board Diagnostics) standard requires.

So, it's also theoretically possible that there's an electrical defect in the lamp itself...
Otherwise, as Matthias mentioned, I would first check the mass airflow sensor (MAF sensor), as it sometimes gives incorrect readings sporadically.

Best regards, Jochen.

Hello,

'Thank you for the quick responses. The manual refers to the lamp as 'exhaust monitoring.' I can rule out a defect in the lamp itself, as the light illuminates with a brief 'jerk.' I will investigate the possibility of a stuck or clogged EGR valve; that's something I had in mind. However, it's really surprising that the workshop shrugs its shoulders when there's a stored error code and doesn't try to find the cause.'

Best regards.
Jean.