Exhaust Leak or Bad Cat?
 

My A4 has recently developed what sounds like a bad rattle under idle that gets worse at some RPM's. The engine is throwing the following codes:


16825/P0441/001089 - EVAP Emission Control Sys: Incorrect Flow

16795/P0411/001041 - Secondary Air Injection System: Incorrect Flow Detected

17834/P1426/005158 - EVAP Purge Valve (N80): Open Circuit

16816/P0432 - Main Catalyst: Bank 2: Efficiency Below Threshold


I replaced the vacuum hose for the SAI system a few weeks ago which makes that error even more annoying...

Attached is a video that shows the noise pretty well.


I'm very much hoping it's not the cat, although that might be easier to fix than tracking down a leak somewhere else down the line...


Any ideas would be greatly appreciated, thank you!

If you are throwing codes it's likely the matrix is breaking down and shaking inside the cat.

Is there a specific RPM range it rattles at?

The first thing I would do is get under the car on jacks and when the car is cool. Go up and down and jiggle the heat shields to see if any are loose. Next start the car up with the exhaust pipe cool. Spray some cool soap water on the flanges and sleeves of the exhaust and check for bubbles. Again cool or it will evaporate before the bubbles form. If that doesn't pinpoint it start trying to isolate the location.

Thanks - I'll try this when the car cools. The noise seems to be loudest between 1,100 and 1,200 and when the SAI is running during a cold start. Sounds like it is in the front half of the car. Luckily live in a state without emissions, but that noise is annoying as heck and so is the CEL...

Will reply with any updates...

For what it's worth, I had an O2 sensor that worked it way lose once. Sounded a lot like that.

Tried the heat shields just now, everything is solid. Started the car on the jacks and took a listen and the sound is definitely coming from the cats themselves... Which sucks :/

As far as I'm aware it doesn't sound exactly like a failed cat, and I haven't noticed any decrease in power, no-starts, or kills, although idling has been more sluggish than usual...

From what I've found, it seems like a broken cat is a symptom of something wrong before the actual cat, and would just lead to the same problem with a new one eventually, is that correct?

If the cat is rattling the biscuit may have broken loose inside.
If you get a piece of hose you can use it to locate the location of the sound. (Like a heater hose 2 or 3 ft long, one end on your ear) If it goes away after the SAI pump shuts off look at the plumbing from the pump to the valve at the back of the head. There are some rubber elbo hoses that fail.

The sound stays after the SAI shuts off, it's just that while it's running the idle RPM is high enough that it's quite a bit louder. I will see if any of the elbows are disconnected...

I am wondering, aside from the failed cat code, does anybody have any tips on diagnosing/fixing the three remaining codes or is it possible that they are being thrown because the cat has (most likely) gone out? I find the EVAP codes a bit confusing in that case, however...

I should also mention that there was a hissing from my gas cap last time I filled up a couple days ago...

I am sometimes amazed at how connected some of these codes are. That said, I can easily connect the gas cap hiss and the EVAP codes, but its hard to connect them to a bad CAT. I think you have two sets of problems.

Hmm.. the N80 just passed an impedance test and I'm getting voltage to the harness. I've cleared the engine codes and will report back with whatever pops back up -- would incorrect flow plus an open circuit be a sign that the entire solenoid has physically failed? I'm not entirely sure how to free the hoses clamping it in so I can't easily test if it will hold a vacuum...