P0128 code even after replacing thermostat and sensor
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2011 C300
P0128 code even after replacing thermostat and sensor
A few months ago, I noticed my 2011 C300 was taking a little longer than usual to warm up. It would reach normal operating temperature (~85-87C) and would be fine at idle. I’d notice a very slight drop to 80C driving around town. However, on the freeway, the temp would rapidly drop to ~70C.
About three weeks ago, the CEL came on, and gave a P0128 code. After reading the forums here, it seemed likely the thermostat needed to be replaced. I replaced it with a kit from Amazon which included the thermostat and coolant temp sensor; I also replaced the coolant at the same time. However, after replacement, the CEL stayed on and the car still took unusually long to warm up and the temperature would rapidly drop on the freeway. I then purchased a thermostat and coolant sensor replacement kit from FCPeuro, thinking the one I bought from amazon might be faulty.
After replacing the thermostat a second time with the FCPeuro kit, I’m still encountering the same issues: p0128 code, long time for engine to warm up, and coolant temperature decreases rapidly when on the freeway.
At this point, I think it’s unlikely to be the thermostat or coolant temp sensor, but I’m not sure what else to check. Any advice would be greatly appreciated! It’s due for a smog check, and with the CEL on, I know it’s an automatic failure.
Edited to add: I forgot to mention that the radiator fan seems to be operating normally. It’s not coming on at idle or around town as far as I can tell.
About three weeks ago, the CEL came on, and gave a P0128 code. After reading the forums here, it seemed likely the thermostat needed to be replaced. I replaced it with a kit from Amazon which included the thermostat and coolant temp sensor; I also replaced the coolant at the same time. However, after replacement, the CEL stayed on and the car still took unusually long to warm up and the temperature would rapidly drop on the freeway. I then purchased a thermostat and coolant sensor replacement kit from FCPeuro, thinking the one I bought from amazon might be faulty.
After replacing the thermostat a second time with the FCPeuro kit, I’m still encountering the same issues: p0128 code, long time for engine to warm up, and coolant temperature decreases rapidly when on the freeway.
At this point, I think it’s unlikely to be the thermostat or coolant temp sensor, but I’m not sure what else to check. Any advice would be greatly appreciated! It’s due for a smog check, and with the CEL on, I know it’s an automatic failure.
Edited to add: I forgot to mention that the radiator fan seems to be operating normally. It’s not coming on at idle or around town as far as I can tell.
Last edited by achromatic; Jun 22, 2019 at 03:04 PM.
Thread Starter
Newbie
Joined: Jun 2019
Posts: 3
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2011 C300
I thought it was the part as well the first time, but after replacing it a second time with a thermostat from fcpeuro and seeing the same issues, I’m pretty confident it’s not the part.
Definitely there could have been issues with the install; I’ve never done it previously, but what’s odd is that I’m getting the exact same symptoms as before I replaced the original thermostat, and again when I replaced it a second time. There hasn’t been any coolant leakage, and it heats up fine at idle, but with any meaningful airflow, the temp drops instantly. Do you know of any other reasons this would happen?
I’m trying to avoid it, but if there’s nothing else that would cause these issues (and I can’t find any, but I’m not at all an expert), then I’ll bite the bullet and take it to the dealer and let them replace the thermostat and hope that does the trick.
Definitely there could have been issues with the install; I’ve never done it previously, but what’s odd is that I’m getting the exact same symptoms as before I replaced the original thermostat, and again when I replaced it a second time. There hasn’t been any coolant leakage, and it heats up fine at idle, but with any meaningful airflow, the temp drops instantly. Do you know of any other reasons this would happen?
I’m trying to avoid it, but if there’s nothing else that would cause these issues (and I can’t find any, but I’m not at all an expert), then I’ll bite the bullet and take it to the dealer and let them replace the thermostat and hope that does the trick.
