P0420 code, which o2 sensor to change on my Tahoe?

**SScatman02** · 11-10-2007, 12:26 PM

Just thought I would run this past some others, im looking to clear up the problem with the P0420 code on my Tahoe, hopefully replacing the 02 sensor will clear the code... But on my tahoe, there are two sensors on the left side...when i had the code read, it read bank 1, but which one should I replace...im thinking the one behind the cat?

Any help appreciated.

**2001NBMZ28** · 11-10-2007, 03:52 PM

P0420 is a cat efficiency code - so probably the post cat sensor since it monitors the cat just like on our cars. Sure it's the 02 and not the cat?

**jigger1166** · 11-11-2007, 06:51 AM

A way to tell if it's the sensor, is to swap it with the one on the other side.

So, if the sensor after the cat. on the driver's side is setting a DTC, try swapping it with the sensor after the cat. on the passenger side. If the DTC sets on the other side, you know it's the sensor. If the DTC stays on the left side, it could be the cat., and might be covered under the emissions warranty.

**SScatman02** · 11-11-2007, 08:23 AM

I am not sure it is the o2, i am just hopefull it is...the tech that checked the code said that normally if it was an o2 sensor it would throw the "P0420 code, bank 1, and a senor 1 or 2" This one just read Catalyst efficiency low-bank 1
-"The oxygen sensors monitor the catalytic converters ability to store oxygen."
-Probable causes
-1 catalytic converter defective (failure possibly due to #2, 3 or 4)
-2 engine misfire or running condition
-3 Large vacuum leak
-4 Engine oil leakage into exhaust valve guide seals.
piston rings.

Thats the rundown the tech at auto zone was able to give me after the reading...

And ill have to switch the o2 sensor to the other side...didnt even think to do that... thanks.

**2001NBMZ28** · 11-11-2007, 08:47 AM

Cats are covered under the federal emissions warranty for 8yrs/80K. GM extended the warranty on the LS1 F cars, don't know if the Tahoe got the same thing. Worth a call to the dealership before spending money on a new one.

**Toneloc71sr** · 11-25-2007, 10:31 AM

Hey Scatman,

I got the same codes. Did you end up fixing it? How did you do it? My local shop wants to replace the cats for $1200. Do cats really go bad? I failed the smog in ca. but havent done a tune up for awhile, can cleaning the sensors and doing a tuneup get me to pass? Guess it wont hurt.

**0rion** · 11-25-2007, 12:29 PM

Originally Posted by Toneloc71sr

Hey Scatman,

I got the same codes. Did you end up fixing it? How did you do it? My local shop wants to replace the cats for $1200. Do cats really go bad? I failed the smog in ca. but havent done a tune up for awhile, can cleaning the sensors and doing a tuneup get me to pass? Guess it wont hurt.

the cats are covered under warranty so if you do replace the cats take it to a dealership. Have you swapped sensors from one side to the other yet to see if the code follows the sensor?

**SScatman02** · 11-26-2007, 04:46 PM

Yea, some news to follow here... I had to get a new engine...the o2 sensors were ragged out due to the engine burning the coolant. The water/coolant vapor blew out the 02 sensor...

My tahoe is at a local shop right now getting a remanufactured engine...ordered with a 3 year 100,000 mi warranty. Same engine as the dealership, but the dealership wanted 4,800$$$$...

A local guy will do it for less...sucks it had to come to this.

**Badd98CamaroSS** · 11-26-2007, 05:30 PM

If the cat goes out, it will either glow red (hot), or you will smell a rotten egg smell, or both. Since the DTC said cat efiiciency, if IS the downstream sensor (after the cat)...if the code had come up as rich/lean, it would be the upstream sensor. The best way to confirm it is the sensor, and not the cat is to switch the sensor to the other side, as had been suggested above...hope this helps.

Joseph