P0420 Code After Cat Replacement

[Donald Lee]

Hi all,

I recently had my catalytic converter replaced as I was having a problem that turned out to be a transmission issue. Anyways ever since this cat has been new on the vehicle (2007 Pontiac Grand Prix GXP), I have been seeing the P0420 code nonstop. The shop stated that sometimes new cats can throw this code out. I have cleared it several times, the shop replaced that cat with another new one and I am still having the same issues. P0420 is the only code in the database. I have not noticed any issues regarding performance just the check engine light always turning on. Is this a possible o2 sensor issue or something that will require a tune?

Thanks

Topic

[Dave Tidman]

You may want to take a look at these 2 videos:

https://www.youtube.com/watch?v=9VZ5K8n5jj0 (ETCG)
https://whttps://www.youtube.com/watch?v=vyVnhCIMDnw (ScannerDanner)

Hopefully you have a scan tool that can read live data, that will give you a direction to look at.

[Will]

Might want to check if there’s a hole in the exhaust or the shop messed up on the weld somewhere. I’ve seen these on Toyotas for this same issue. The code means a “converter low efficiency” Fault code. It could mean a bum converter, bad 02 sensor upstream or downstream or an exhaust leak. Since the shop just replaced it tell them to put it up on the jack with the car running and see if gas is escaping anywhere around where they welded it up.

[Will]

Here’s some reading on the code:

http://www.aa1car.com/library/p0420_dtc.htm

Check for leaks, check that the 02 sensors are responding correctly. If not it’s probably the cat they put in is not compatible with the vehicle.

[Joe]

It could possibly be an oxygen sensor. I recommend that you (or anyone else who drives a car) get a code scanner, preferably one that shows live data. Here’s a nice low cost scanner that works great:
http://www.amazon.com/Buke-U581-OBDII-EOBDII-Scanner/dp/B00S5R56B8/ref=sr_1_1?s=automotive&ie=UTF8&qid=1438155973&sr=1-1&keywords=Blusmart+U581+CAN%2FOBDII%2FEOBDII+Memo
While possible, it seems unlikely that an O2 sensor would just happen to go bad right after the cat swap, not to mention that typically if the sensor goes bad you’ll get a code specific to the sensor rather than the P0420 code. Check what changed, which in this case pretty much means check for exhaust leaks. Otherwise, like Arcturus already mentioned, it may be the part that the shop is using is not compatible. You might have to shell out the $ for an OEM catalytic converter; aftermarket ones are cheaper for a reason, and in this case the reason isn’t just dealer markup.

[Donald Lee]

Ironically enough the second time they replaced the cat, they didnt finish the weld. A noticeable leak was there but since fixed. I was curious about the o2 sensor going bad, but wouldn’t there also be another code related with the P0420 such as a faulty sensor code? The only other choice I have it seems is to take it to a shop as I can’t seem to find a scanner doing live feed anywhere without purchasing it.

[Author]

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