Catalyst Fail: Updated

[R490]

So, I've been going through gas at an alarming rate. Something to the tune of ~150 miles to a tank.

After running the readiness test on VAG Com again, but under the proper circumstances; the only thing that was listed as failing is the catalyst/catalytic converter.

However, there are two codes (17521, 17539) which refer to 02 sensors which also may be what's causing the low gas mileage. In other words, I have no idea what needs replacing and what should be left alone.

Also note that I haven't had a CEL from the 02 sensors, which makes me think that they're possibly fine.

[a4kamila6]

If 02 sensors are shot, ecu by default will run richer as a safety precaution so you dont run too lean and mess your engine up. I would think thats why you may be burning more fuel.


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[R490]

If 02 sensors are shot, ecu by default will run richer as a safety precaution so you dont run too lean and mess your engine up. I would think thats why you may be burning more fuel.


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The idle seems fine, except the car revs up to 1.5 or so when I start it in the cold.

[carmudgeon]

Both of those codes are in reference to the front O2 sensor on each bank, which is what the ECU uses to compute fuel trim. If they've failed that would explain the poor fuel economy.

[rollerton]

If 02 sensors are shot, ecu by default will run richer as a safety precaution so you dont run too lean and mess your engine up. I would think thats why you may be burning more fuel.

^ This.
Replace 02 Sensors, probably downstream ones. Pretty rare that BOTH cats just decide to fail- it's not like they're mechanical devices. They can be all busted up inside and still technically do their job and clean the exhaust to some extent.

[R490]

Ah, thanks guys!

Are those two DIY-able?

I've got fair mechanical knowledge, but I'm used to working on Japanese cars. Even though I've never owned a Japanese car....

[estomax]

if your car is running too rich that will mess up the catalytic converters over time, you can replace the o2 sensors they are a bit of a bitch to get to but you can get to them from above, need extensions, ujoints and a o2 sensor socket.