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Hello, I too am faced with most likely replacing my catalytic converters on my 08 GTS with 128K miles. I have P0421 and P0431 codes. I already replaced all 4 O2 sensors, but the codes came back. So far, I've replaced thermostat, thermostat housing and updated the coolant lower pipe in the process as well. I've searched and ran out of other options on how to get rid of these codes. I'd appreciate any other suggestions or expertise anyone may have on this matter. If you could share some workshop instructions on performing this task, thank you.
Hi Galun, I’m sorry some people don’t know help when it’s offered. I’d like to take this opportunity to ask for your help if you’re still inclined to do so. I also have a ‘08 957 GTS and been chasing these codes since I got it, 0421 and 0431 at 124K miles. Is it your experience that it’s likely the cat that need to be replaced? I recently had the thermostat and housing plus the coolant tube replaced since it had a code for an open thermostat, and I was told by a local P indy that the cat is not warming fully because of that. Unfortunately, 0421 and 0431 still showing after thermostat replaceme