While diagnosing a chronic issue with our '99 A6 Quattro sedan, I got some confusing results when I logged a cold-start drive cycle using my VAG-COM. When I play it back using the Ross-Tech Scope app, I get the following:



The green line is the O2 sensor for bank 1, and the yellow line is the O2 sensor for bank 2 (both as a percentage). While both sensors appear to respond similarly (the peaks and valleys more or less match up), why is the yellow line constantly sitting higher than the green one? Shouldn't they both be sitting at about the same height?

Looking at the raw numbers seems to confirm this. The numbers for bank 1 are always "within spec" (barely), but they're also always lower than the numbers for bank 2.

The impression I get from this is that:

- both O2 sensors are working fine (they both have about the same peaks and lows)

- each bank is running with different levels of fueling

Is my assessment correct? Or am I way off the mark?

 

What is the chronic issue you are trying to figure out? What are the fault codes?
As far as the placement of the lines , they are given there own separate space. What you would look for is a varying of voltages from one sensor to another ,ot where they are placed on the screen.

 

That doesn't sound right. As I mentioned, the raw numbers reflect the different heights in the lines. The bank 1 numbers are in fact consistently lower than the bank 2 numbers. The graph displays this accurately. Both sensors start out at the same level and immediately diverge once the engine is started.

I'm getting many different error codes, mostly randomly. They include:

P0300 - random/multiple cylinder misfire detected
P0301 - cylinder 1 misfire detected - intermittent
P0302 - cylinder 2 misfire detected - intermittent
P0303 - cylinder 3 misfire detected - intermittent
P0304 - cylinder 4 misfire detected - intermittent (this one's more rare)
P0305 - cylinder 5 misfire detected - intermittent (this one's more rare)
P0306 - cylinder 6 misfire detected - intermittent (this one's more rare)
P0411 - secondary air injection flow too low
P0422 - main catalyst bank 1 - efficiency below threshold
P1338 - camshaft position sensor - open or short to plus - intermittent (this one's new)

I occasionally get strings of misfires while idling or driving. Especially at light throttle. All spark plugs have been replaced, all coil packs have been replaced. We've gone through every standard diagnostic procedure without any luck. The readiness test did not reveal any problems. Changing the spark plugs helped eliminate the misfires at idle after a cold start, but the codes still come up anyways, and the misfires still happen. At this point we've pretty much eliminated the ignition system and have moved on to fueling as a possible cause. This has been driving me insane for 3 weeks.

 

You will need to find out what is causing your misfires.
Looking at o/2 sensor data when you have misfires is not going to help you.

Look at the misfires in the data blocks.
What rpm's are the misfires occurring?Idle , acceleration? Look at the freeze frame data.
Check for vacuum leaks , check the MAF readings.

 

well... yeah

Already did that

It can happen at any RPM, it's very random. Most likely at light throttle while driving up a hill, but it happens under all conditions. Freeze frame data has given no useful information.

Already did that, replaced all vacuum lines and checked for vacuum leaks at all locations. No vacuum leaks detected anywhere, old lines that were prone to collapsing under vacuum have been replaced. MAF reading is the orange line in the above chart, I've been checking that too. Everything checks out within spec.

The really bad misfires appear to be concentrated on bank 1, usually cylinder 2. This is why I'm now looking at fueling as a possible cause, and this is why I'm currently looking at O2 sensor readings.

 

OK , what are the fuel trims for bank 1 & bank 2? Is the ECM trying to add or subtract fuel?

 

Neither. When I checked last week, the fuel trim was the same on both banks, and perfectly within spec. I'll confirm that in 30 minutes when my wife gets home with the car. It currently has 7 of the above-listed error codes active, all since yesterday.

 

I don't know if you have the capability but a back pressure test on bank 1 would be a good idea. Especially with a cat efficiency code.
I have seen partially clogged cats cause misfires before.

 

That's what I've been wanting to do for over a week, but I don't have the capability to do it myself. It also sounds like the cat is rattling when I accelerate from a stop, which supports that theory. It might also explain the uneven O2 sensor readings (backed up exhaust and injected air at that sensor).

I bought the car 3 weeks ago from a mechanic. He's promised to get it in tip-top shape and get rid of the codes, but it's taking forever. I'm sick and tired of taking the car in to be looked at, have some random thing fixed, and then drive it around for a day only to have the engine light come back on and the misfires strike again. So I've started diagnosing it myself to try and find the problem.

The mechanic seems to be purposely avoiding the cat as a likely cause, probably because that would be the more expensive repair. Every time I suggest it as a cause, he dismisses it as "impossible". Drives me nuts. Eventually it'll be the only remaining possibility...

 

The good thing about the Vag-Com is you can set readiness when done to confirm repairs were made correctly. The shop you are/were bringing it to might not have that capability or know how so after a day or so of the monitors running the light will pop back on if all the faults are not repaired.