P0420 and P0128 - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 4 (permalink) Old 12-19-2017, 09:10 AM Thread Starter
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P0420 and P0128

Hello Fellas,

I have two unrelated issues that I am hoping are related. I have a 2005 Cobalt, 2.2, manual trans with 163k miles on it and have low coolant temps. This is an obvious and easy fix, change the thermostat. For example, this morning it was about 41 degrees outside and my coolant temp hovered around 165 at 65mph. It usually hovers around 175 with temps between 55-80 degrees at highway speeds. It will creep up to 215 degrees when idling after a while.

Recently I have had a P0420 pop up as well and have been doing a bit of research trying to find out what I need to do. There is a lot of mixed information out there and I am trying to decipher it all. I don't want to just start throwing parts at the car since its my commuter vehicle and I am not planning on keeping it for ever.

Here is my question, can low coolant temps prevent the cat from getting hot enough to perform its function properly? I am hoping this is a possibility.

I have been keeping an eye on the o2 sensor readings with a scan tool and the downstream sensor is jumping around more than I think it should. This is leading me to think that it is leading to a bad cat. If it is truly a bad cat, I have a feeling that it is a symptom of another/larger issue.

My last issue is oil consumption/loss. This weekend I checked my oil levels and noticed it was below the dipstick. I ended up having to add 2 quarts of oil. I do not have oil stains on my drive way or any visible oil leaks. I bought the car with about 158k miles on it and was told it was just changed, which seemed to be true because the oil looked clean and full.

Any thoughts on the above would be appreciated.

newb - 2005 Cobalt, Base Model, 5 speed. 2.2 Ecotech
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post #2 of 4 (permalink) Old 12-19-2017, 12:21 PM
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I would start by changing the thermostat it is obviously stuck open or the rubber seal has collapsed and not letting it warm up fast enough. This can lead to both your other symptoms, oil consumption and CAT HO2 correlation errors. Keep and eye on the oil since these Ecotec prefer a full mark to keep the timing chain oiled., I'm not saying you may have damaged the chain yet but every time you fill with gas and your car is on a level plane you should check the oil. Don't follow the oil meter if you are using dino oil.

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post #3 of 4 (permalink) Old 12-20-2017, 08:42 AM Thread Starter
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thanks Coby7, I am going to try and replace the thermostat this weekend and report back after some time of driving the car.

newb - 2005 Cobalt, Base Model, 5 speed. 2.2 Ecotech
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post #4 of 4 (permalink) Old 12-30-2017, 10:27 AM Thread Starter
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Just to update, I have changed the thermostat and cleared the codes. So far I have driven over a 100 miles and the P0420 hasn't tripped the CEL yet. I am pretty sure it will pop again though.

newb - 2005 Cobalt, Base Model, 5 speed. 2.2 Ecotech
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