My 04 SS currently has two codes for the CEL, a P0128 and P0420. The OBD-codes.com website and one or two previous posts I could find on this site seem to indicate it's possible that a faulty coolant temp sensor can cause the P0420 code. Does this seem consistent with other people's experience? If that's the case, and since replacing either of them requires draining coolant, should I just replace the thermostat and coolant temp sensor since they are, relatively speaking, cheap?

 

I have never heard of a tstat or temp sensor being related to a P0420 code. That is typically for a failed cat OR you replaced a cat with a non-oem cat and the PCM believes the cat is bad. It can also be a failed downstream O2 sensor.

The P0128, that does not ring any bells with me.

FYI - the change a Tstat or a coolant sensor, you do NOT need to drain the entire cooling system. You can either make a mess of coolant and top off what you lost or you can drain a little out (but no guarantee you will drop the coolant level where you are working).

If you are going to replace the temp sensor, I would just do the Tstat at the same time. A Tstats wear out with age and if you don't know the age of what you are using now, for an extra $5-$10, change it and have piece of mind.

 

P0128 means the car isn't getting up to temperature fast enough, probably a stuck open thermostat.

P0420 is typically a cat problem, and they are common for our cars

 

I agree with both your assessments, I just found it a little coincidental OBD-codes.com basically listed P0128 as a cause of P0420 so I figured I'd ask around. Temp sensor and t-stat are easy enough, I'll start with them.

Is it basically a guarantee 0420 is a failed cat and not the sensor? It doesn't "rattle" and I've noticed no decrease in power. I've been looking for an excuse to just remove the cat (I live in non-emission state) and put in a ZZP downpipe. Do I need 2.5" or 3"?

 

The P0420 can be a cat, failed rear/downstream O2 sensor or the car does not like the readings of a non-oem replacement cat. Usually when the cat goes bad, you are also having performance issues with the car, so it could be just a failed downstream O2 if you have the OEM cat and no performance issues.

If you are looking at a ZZP downpipe and high-flow cat, for a non-Supercharged 3.8, go with the 2.5", for the Supercharged 3.8, go with the 3".

Now, if you get the ZZP DP with high flow cat, you stand a good possibility of popping the P0420 code (even if the O2 sensor is good). Two common fixes for this:
- Using spark plug non/anti-foulers to space the O2 out of the exhaust stream (it's the cheapest remedy, less preferred, as it's a visible and stands a chance it *might* not work, but for the cost, worth the try). But if the downstream O2 is bad, this probably won't work.
- Have the PCM re-programmed to erase the P0420 code so it never pops up again. It's a guaranteed remedy. Downside is, it normally costs more to have this done.

 

Thanks, Maniac. I'll hold off a little bit on the cat then seeing is how I don't have performance issues yet and I still have the OEM cat. I was actually going to go ZZP without a cat and try to space out the O2 with the anti-foulers as you suggested.

I noticed you didn't mention O2 emulators as a solution? I take it you're not sold on the concept?

Do I need the gaskets from ZZP when the day comes I order it?

 

Per all websites that I have seen the O2 emulator sold at (including ZZP), it does not work on an '04, therefore it's not even an option (at least for my car, since I have an '04).

I would definitely go with new gaskets.