2000 A6, 2.7t, 156,000 miles

My EPC dash light came on about a day after replacing my inner and outer CV boots on the driver's side and doing some engine work where I pulled some plugs and moved hoses and connections out of the way to tighten valve cover nuts. I did leave the Y-pipe PCV connector off for a day and drove it that way before noticing. A few days later the check engine light came on.

One other thing to note: A few weeks prior to seeing the dash lights and codes, my oil temperature started reading low. This happened right after changing the oil, using the usual 0W-40 Mobil 1 synthetic. It always read about half way up the gauge, and now it struggles to get to 1/4 of the way, and usually runs at 1/8. It also takes a lot longer to move up than it used to.

Here are the current fault codes:


VCDS Version: Release 10.6.5
Data version: 20110930
Craig's A6 2.7t
Monday,10,October,2011,20:09:45:50532

Chassis Type: 4B - Audi A6 C5
Scan: 01 02 03 06 08 15 16 17 18 22 34 35 36 37 45 55 56 57 65 67
75 76 77
--------------------------------------------------------------------------
Address 01: Engine Labels: 078-907-551-AZA.lbl
Part No: 4B0 907 551 K
Component: 2.7l V6/5VT G 0005
Coding: 06612
Shop #: WSC 30011
VCID: 2C4A0B08F690
--------------------------------------------------------------------------
Address 01: Engine Labels: 078-907-551-AZA.lbl
Part No: 4B0 907 551 K
Component: 2.7l V6/5VT G 0005
Coding: 06612
Shop #: WSC 30011
VCID: 2C4A0B08F690

3 Faults Found:
18084 - EPC Warning Lamp (K132) Circuit: Electrical Malfunction
P1676 - 35-00 - -
18098 - Malfunction Indication Light (K83): Electrical Malfunction
P1690 - 35-00 - -
16815 - Warm Up Catalyst; Bank 2: Efficiency Below Threshold
P0431 - 35-00 - -
Readiness: 0000 0101

--------------------------------------------------------------------------
Address 36: Seat Mem. Drvr Labels: None
Part No: 4B0 959 760 A
Component: Sitzmemory D05
VCID: 2B480814F39E

1 Fault Found:
00668 - Supply Voltage Terminal 30
07-10 - Signal too Low - Intermittent


The seat memory has not worked for over 100,000 miles and that code has been there for that long. I've spent some time diagnosing that problem, but as I'm the only driver, this is not a priority to me.

P0431 has shown up intermittently. It was not on the initial scan with P1676 and P1690 after the dash lights came on, but was there with this latest scan.

Things I have checked so far:
1. All engine wire harness plugs, hoses, air box clips and connections.
2. Removed APR ECU, replaced with stock ECU, cleared codes, ran car and re-scanned. Dash lights and codes returned. Put APR ECU back in.
3. Removed instrument cluster to check bulbs. Discovered that 'bulbs' are surface mount LEDs. Re-installed cluster.
4. Cleaned battery connections. Load tested battery = good. 13 volts at rest, 10.5 volts under load held for 15 seconds, 14.5 volts while running (alternator).

An observation:
Today on the way to work it was 62° and rainy. When I started the car this morning, the check engine and EPC lights came on as usual. The lights stayed on while driving 30mph to 50mph, but while idling in traffic, the CEL faded to very dim and went off. It occasionally flickered and came on brightly for a second, but then faded so I could barely see it and then went out completely. Once I got back on the highway, the CEL came back on, though not 100% brightness and it stayed on until I got to work 20 minutes from home. EPC has stayed on 100% since last week.
The weather has been sunny and clear since the CEL came on until this morning, and this is the first time it has gone out intermittently. It just happens to coincide with the weather, though it may be completely unrelated.
Question: is the CEL designed to fade on/ off like mine is, or is it a 100% on / 100% off indicator light?

Other recent work (background information): Auxilary water pump, timing belt/ water pump/ drive belts/ tensioners/ cam tensioner gaskets/ spark plugs/ valve cover gaskets, full front suspension control arms, etc replaced, front subframe mount bushings, left front inner/ outer cv boots, coolant temperature sensor.

The car runs normally and the same as it did before the lights came on the cluster and the codes in VAG-COM.

As I don't want to throw parts at this without having reasonable assurance they are bad, can anyone recommend a good diagnosis path going forward? I am not sure where to go from here.

Thanks

 

TTT

anybody?

Are there specific systems to look at related to the EPC and malfunction indicator light (MIL) codes?

I have done a readyness scan with VAG-COM and 02 sensor resistance was constant between all of them, and nothing else came up in the other systems to throw up a flag.

At this point, anything I look at will be more or less guessing.