I have a 2003 A6 Quattro 3.0L. Some time ago, the check engine light came on - I have one of those simple VAG readers so I plugged it in and it game me the following DTC codes:

Engine:
16805 Warm up catalyst; Bank 1: Efficiency below threshold.
19537 Engine temperature too low. (fan on all the time?)
17544 Long Term Fuel Trim Add.Fuel,Bank1 System too Lean (intake leak?)
17546 Long Term Fuel Trim Add.Fuel,Bank2 System too Lean (intake leak?)
16486 Mass or Volume Air Flow Circ Low Input
17550 Load Calculation Cross Check Lower Limit Exceeded
16684 Random/Multiple Cylinder Misfire Detected
16685 Cyl.1 Misfire Detected
16688 Cyl.4 Misfire Detected
17887 Brake boost Vacuum System Mechanical Failure
16687 Cyl.3 Misfire Detected
16686 Cyl.2 Misfire Detected
16689 Cyl.5 Misfire Detected

A/T:
17095 Transm.Fluid Temp.Sensor Circ. Range/Performance

I know that one code can cause another, which causes another etc. so if anyone can decipher this and point out in the direction of a likely problem...
At this point, the A/C blower fan is always on (a mom and pop shop said the blower motor relay died) which from googling may cause 19537.
I recently changed the air intake filter myself - could this be part of the problem if it has a leak after I connected all the parts back?
The misfires codes showed before I changed the filter, although only 3.
Once in a while, the ESP light lights up and when in drive the car holds high revs even on low speeds and the engine dies ones in a while when making a turn. When the light is on and in park, pushing gas a little bit and releasing sends the revs needle upwards 3-3.5k and then dropping to 0.25-0.5, with most cases sending it to 0 after that and the engine dying.
When the ESP light is not on, the car still has fairly high revs before switching to the next speed. E.g. I noticed that it would steadily hold 3k revs at 40mph but if I give it a bit more, up to 42-43mph, it switches and drops down to 1.5-1.7k revs.
Any suggestions, comments, advises are much appreciated, before I go and spend a lot of money at a dealership to just diagnose it...

Thanks!

 

clear the codes and see which come back.

the first one is a common problem for the 3.0 and you will need a new catalytic conveter.

the second one can probably be elimanited with a new coolant temperature sensor.

the 3rd 4th and 5th might want to START with cleaning the MAF or replacing it. and im pretty sure those missfires will be gone by the time you fix the ones i mentioned. (given your coil packs and sparkplugs are OK.)

unplug the MAF and see if the car runns better. most likely the faulty MAF is causing you these troubles

have you done a transmission fluid change?

 

I cleared the codes yesterday. The revs were going up and down for about a minute or so and then steadied to normal level. Checked the codes and it came up with 16502 - Engine Coolant Temp.Circ High Input. I cleared that out again. After that the check engine wasn't on and there was a definite difference in how the car started handling itself - the revs weren't going up that much and all actually seemed more or less good. Today, however, check engine came back on and the revs constantly go slightly up and down, it feels like the car is rocking a little bit. Just checked the codes again and it came back with this:

19537 Engine temperature too low.
16684 Random/Multiple Cylinder Misfire Detected
16685 Cyl.1 Misfire Detected
16686 Cyl.2 Misfire Detected
16687 Cyl.3 Misfire Detected
16688 Cyl.4 Misfire Detected
16689 Cyl.5 Misfire Detected
16690 Cyl.6 Misfire Detected

I don't have a chance to get under the hood to check MAF until at least tomorrow evening, once I do I'll post how that worked.
I haven't done transmission fluid flush since about 06-07 - the level looks ok. I did a top off of the coolant recently, though, as it was below min level.

 

With so many codes, it's tough to tell. But those misfires on every cylinder are unusual OR a sympton of coil pack problems. There was a TSD for bad coil packs on my 2003 A6 2.7T and 2003 A4 3.0. The dealer initially said they were not covered, but when my A6 coilpack died at 192k miles, I brought it to a different dealer who said it was covered and replaced all the coil packs. Call your dealer and find out. If you need a copy of the TSB, email me at jjaiven@chartermi.net.