OK well the P0420 came back today. I have put in a new thermostat from when I had both P0128 and P0420 codes a few weeks back. I'm wondering if it was the coolant temp sensor all along?

The common denominator between these two codes was the coolant temp sensor.

So now I only have the P0420 sensor at this time back but I'm wondering if I should swap out that coolant temp sensor? Or is there more diagnostics I should focus on? Those sensors are not real expensive.

 

Anyone? So what would be the most likely cause of this P0420 code returning? I'm planning on getting a free exhaust leak check to eliminate that but than what would be the likely culprit? I'm thinking the coolant temp sensor?

 

Like stated earlier, the P0420 is actually a converter code. Other things can cause it, but it could actually be the converter too. Maybe try a product like CataClean? If the temp sensor isn't giving erratic readings, I'd say its fine.

I like the direction this thread took regarding Dex-cool. I'm so tired of people blaming it for everything. I always said that DexCool, if exchanged at the same interval recommended for green coolant, will never give you trouble. I still not convinced it is any different than green. There are so many products on the market (Prestone, Celsius) that are mixable with both, so how different can they be?

 

You may be on the right track with the coolant temp sensor, but if you are losing coolant (and it is internal) coolant could be getting into the cylinder(s). Unspent chemicals (like glycol) can enter the exhaust stream and coat the cat converter. The result is a P0420 code. I think I would backtrack a bit and conduct a cooling system pressure test and/or a cylinder leak down test to make sure you don't have a bad intake or head gasket.

 

I actually have a new catalytic converter that isn't even a year old installed. I had it installed back in June or July of 2013.

Ugh I think you might be onto something actually. I have been losing coolant and its winter here and I don't see any leaking so I'm wondering "where" the coolant is going.

I was dreading the answer but I think I might know now.

Would Cataclean work for this issue?

 

CataClean would only be a patch and the converter would foul again unless you fix the internal leaking. Some rad sealer might help. Some people hate rad sealer

 

Yeah I've actually used Bar's with great success. I have used it in about 3 different cars starting with a '91 Sunbird, a '92 Talon TSI Turbo, and a '00 Malibu. All had some type of radiator leak and it actually worked in all of them with no plugged heater cores or ill effects.

I'll probably end up trying some.