I have a 2003 A4 convertible 3.0 that has the engine check light on intermittently.
A few weeks ago I went to AutoZone and Error code P0421 was on (Warm up catalyst efficiency below threshold – bank 1). Then the light went off. Then it came back on and I thought it would be the same code and did not bother to check it up. The car felt good, no vibrations, around 29 mpg highway and passed the emission test so I decided not to investigate further.

Last week I had all 6 ignition coils replaced at no cost by Audi (Audi recall). The car seemed to be less nosy (although maybe it was just my imagination). After a couple of days I noticed that it took longer to star up the engine. Then, the ESP light came on. The car took longer to start, stalled a couple of times at red lights, jerked when starting in step slopes and the rpms dipped to around 500 when stopping, then rebounded to around 1500. I went immediately to AutoZone and they found the following codes:
P0102 (MAP (Man. Absolute pressure)/BARO or MAF (Mass Air Flow) / VAF (Vane Air Flow) sensor condition
P0172 (Fuel Trim Bank 1 condition)
P0175 (Fuel Trim Bank 2 condition)
P0420 (Catalyst efficiency low-bank 1)

I called Audi because I thought it could be something to do with the new coils and got an appointment. But yesterday I had to take the car for a short spin and the ESP light was off. No more issues with the rpms and the engine starts quickly. The engine light is still on.

Can anybody advise what to do / what to check / replace first?

I have read the Common problems + repairs sticky and it looks like this
2. CTS sensors, the black sensors are faulty, and will cause gauge malfunctions, increased fuel consumption, rich fault codes, excessive cranking times when the engine is warm or cold, and even random misfire faults. Replace with a green top sensor to cure these issues.
Could be the issue. If so, what are the CTS sensors?

Could the ESP light be on due to the other issues with sensors or ECU or ...?
Thanks!

 

romar1086: Did you solve this problem? I have a 2003 A6 with a 3.0 engine. I have all the same symptoms that you described. In addition, the first time this happened to me (2 days ago), the "coolant system error" symbol came on the display briefly. I had that error happen to me once before on an older A6, and I remember changing the engine coolant temperature sensor (CTS) for it, so I am already suspecting the CTS. I plan to do change it for this A6, but I was curious to know if that is what you had to do, too. Right now the car is running OK, but only 4 hours ago, I couldn't even get it to start (or at least hold an idle).
Thanks