View Full Version : P 420 code-what does it mean?

Harry Yarnell
11-13-08, 06:14 PM
I'm getting a P420 code, and the shop manual isn't too clear what's wrong. Any help?
1998 Eldo.

11-13-08, 06:27 PM
P0420 - Catalyst System Low Efficiency
Probably a bad CAT.

11-14-08, 07:59 AM
It means you have a bad converter.
Before you replace it make sure whatever caused it to fail has been fixed.

11-14-08, 11:22 AM
Yeah..bad cat. I get it at least a few times a week now, and I recently heard the other morning when I started my car a little bit of rattling..sounds like it's coming apart. If you replace it, let us know what cat you use. I'm curious as I've needed a new one for quite some time now.

Harry Yarnell
11-14-08, 06:57 PM
Yea, that's what I thought (bad cat). I've gotten the best pricing on Northstar cats in the past from Advance Auto.
Now, I've cleared the code (no other action taken) and run this raskal down I-95 to get a good run in. Code has not reappeared. Yet.
This car had bad head gaskets when I bought it (5 months) before I did the headbolt insert treatment. It has since passed maryland emissions testing.

Fire and Ice
11-21-08, 05:49 PM
P0420 is most likely a bad cat, but can also be other things. Grab a Tech 2 or another scan tool that will allow you to monitor the sensors. Watch the upstream and downstream O2 sensors while holding the throttle at about 2000 rpms. If the downstream (post cat) sensor moves around like the upstream (pre cat) then you likely have a bad cat. You might also consider checking to see if you have an exhaust leak (gasket, cracked pipe or manifold, etc) That could also cause the system to register a P0420 code.

11-23-08, 08:11 AM
An amazingly complicated (and incredibly unlikely) set of circumstances would have to come together to create a "false" catalyst code.
Bad O2 sensors set O2 sensor codes, not faulty catalyst codes and the O2 sensor diagnostics must run and pass first before a P0420 diagnostic will even run. The self tests for the O2 sensors are pretty comprehensive and on GM cars, unless you abuse them with extra chemicals in the fuel, or ignoring the check engine light that is on in your face for months at a time they usually last a good long time.

The O2 sensor readings have to be very specific for the ECM/PCM to determine that the catalyst is not working. Those don't happen by accident.
It is good not to jump the gun and throw a part on, the root cause of the failed catalyst should indeed be eliminated before it is replaced.

04-14-09, 12:46 AM
what happens if the cat is hallowed out and the p420 code still exists, since the p420 code is catalyst system low efficiency. When i removed the cat, it appeared to be mostly clogged. This should ultimatly remove the blockage right? i know the O2 sensor has its seperate code, but could it be giving it false readings or be covered with residue from using bad gas. This is on a 2000 sls

sorry for using your forum, but i beleive its a similar problem

04-14-09, 09:50 AM
The pre- and post-cat sensors compare what goes into the cat and what comes out. If the post-cat sensor does not like what it "sees", then it sets a code. A gutted or non-existent cat will continue to set a code, and maybe more.

Look at www.easterncatalytic.com - their OEM drop-in replacements are all high flow units.

07-25-10, 02:48 PM
where the sensor is located thanks

07-25-10, 03:31 PM
morocco, Move this question up to the Escalade Forum ^^^^.

All of these vehicles have different sensor locations and exhaust systems.