The dreaded P0420...

[PurdueB8A4]

Hi all,

Just driving home from work when I got a check engine light-- hooked up the diagnostic & found it was the P0420 (catalyst system below efficiency bank 1).

Car has ~95k miles with only ~35k miles on a new long block put in by Audi. I will say it seems like my fuel gage is going down a lot faster than normal... I used to get close to 400 miles from a tank & now I'm getting around 350ish. I use Shell V-Power (93 octane) Does this seem like it would be a spark plug, 02 sensor, or cat converter issue? No weird rotten egg smells, the cat looks normal, isn't glowing or anything.

Just trying to trouble shoot this before replacing a bunch of parts that possibly don't need to be replaced. Is there a sure fire way to tell which component is causing the problem?

Also, has anyone had any luck with an additive such as Cataclean? Someone recommended running a bottle through the tank when down to 1/4 tank.

Thanks in advance!

[mr_milo]

P0420..... sadly appropriate for today. Hope you get it sorted out.

[PurdueB8A4]

P0420..... sadly appropriate for today. Hope you get it sorted out.

Haha, no kidding... My car is playing a nasty joke on me.

[van462]

Do you just have a code reader or can it read sensor data? It may help in narrowing it down. Front O2 sensor lambda should be near 1 at idle. Fuel trims should be single digits the closer to 0 the better. These numbers help establish that everything it running properly. Then look at the rear O2 sensor voltage output. Normal is near .45 volts(theoretical as I never checked mine with the stock cat),mine reads .6 with a High Flow Cat installed.

[PurdueB8A4]

Do you just have a code reader or can it read sensor data? It may help in narrowing it down. Front O2 sensor lambda should be near 1 at idle. Fuel trims should be single digits the closer to 0 the better. These numbers help establish that everything it running properly. Then look at the rear O2 sensor voltage output. Normal is near .45 volts(theoretical as I never checked mine with the stock cat),mine reads .6 with a High Flow Cat installed.

Awesome, I'll see if I can grab a vag-com & test it out. I'm
Hoping it is just an O2 sensor... Fuel economy has been getting pretty bad. Either that or spark plugs..

[wfn]

Not sure if it's going to be relevant but I used Cataclean on my BMW 530i w/ p0420/430 codes and it didn't do anything, YMMV. Did two cans of it in succession.

[kt883]

it cant hurt to change out the plugs with new ones. I got my plugs for $13CDN from carquest which was half of what dealer wanted. Easy DIY

I have the same P0420, I bought a used cat and I will be swapping myself

It gets damaged from either oil overfill, possibly severe oil burning engine, or leaky injectors which are a known problem for our cars covered by extended warranty for that but you'd know if you had that.

[o1turbo30v]

You need a new cat, don't waste your money on anything else.....i believe Federal Law states these must last 8 years i think (check your manual), i would not clear the code, instead take it in to your local dealer, you should be able to get a free one.

[PurdueB8A4]

I didn't clear the code-- the CEL did disappear however... Weird. I do have an appointment with my local dealer, maybe they can shed some light-- I'm thinking O2 sensor issues.

[PurdueB8A4]

You need a new cat, don't waste your money on anything else.....i believe Federal Law states these must last 8 years i think (check your manual), i would not clear the code, instead take it in to your local dealer, you should be able to get a free one.

I also saw this while searching, but isn't the federal law 8 yr/80k miles, whichever comes first? I just cracked 95k, so I doubt I'd qualify for a "free" converter unfortunately.

[PurdueB8A4]

Can anyone confirm that replacing the stock cat w/ a high-flow unit will definitively cause a check engine light? Wanting to weigh my options-- dealer "confirmed" that the p0420 was due to the cat needing to be replaced -- at $1800 parts & labor (yeah, right).

[punkalex1]

i just installed a 034 motorsports HFC without an 02 spacer and got a CEL after 50 miles. so yea definite CEL