Looking for advice on what to troubleshoot. My 2021 1500 with the 3.6 will sometimes throw a P0128 code (low coolant temp). I run my scanner to clear it and it goes away. After doing this a few times I have noticed a few things on when that code returns and It only comes on when I am running my AC during warm days. I have also notice that the coolant temp does not go over 185 degrees no matter what engine speed it's at or if the vehicle is moving fast or slow when the AC is running. I have also noticed that the cooling fan is running at extremely high speed and does not seem to turn off when the AC is running. When I meet all those scenarios, it will throw the P0128 code. Google searches indiciate bad thermostat, bad coolant temp sensor, possible radiator issues and other common suggestions. I don't think it's any of those, but leaning towards why the fan runs so fast and does not turn off and the temp never goes above 185 with the AC on. Any suggestions on what to troubleshoot? Vehicle only has 51K.
5thGenRams Forums
Register a free account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members through your own private inbox!
P0128 code only when A/C on.
- Thread starter Apny2nv
- Start date
Richard320
Spends too much time on here
- Joined
- Aug 3, 2019
- Messages
- 2,213
- Reaction score
- 2,367
- Points
- 113
Sounds like a bad thermostat. Once the engine warms up, it shouldn't dip below 195 (or maybe 203, not sure on V6) regardless of AC and/or cooling fan. I'd say it isn't closing completely when it should and allowing too much water to get past. With the fan blasting, the radiator gets really cool, so the engine cools down too far.
c3k
Ram Guru
- Joined
- Jul 22, 2020
- Messages
- 1,141
- Reaction score
- 1,268
- Points
- 113
- Age
- 65
Obviously, the fan shouldn't run if the coolant is too cool. (The exception being if AC is on, to pull air across the condenser.)
If the sensor is bad, it could be completing the fan control circuit when it shouldn't, causing the radiator to be too cool.
My thought, if the thermostat is stuck open, the fan wouldn't run. This, to me, points to some sort of bad sensor.
I wonder what module (BCM or ECM?) controls the fan operation?
You should be able to find info on how to test the sensor (what ohms at what temp) somewhere.
If the sensor is bad, it could be completing the fan control circuit when it shouldn't, causing the radiator to be too cool.
My thought, if the thermostat is stuck open, the fan wouldn't run. This, to me, points to some sort of bad sensor.
I wonder what module (BCM or ECM?) controls the fan operation?
You should be able to find info on how to test the sensor (what ohms at what temp) somewhere.
Richard320 and c3k both have good points. To c3's point, the AC is running when the CEL triggers. Does the fan automatically turn on full speed when the AC is turned on regardless of the temp? The dealership recently replaced the oil cooling housing under warranty, but did not tell me until the repairs were done. Had I known they were going to do that, I would have had them replace the coolant temp sensor as I don't think the sensor would be part of the housing like th A/M housing that dorman sells. I'm guessing the easiest thing to rule out would be to replace the thermostat for now. Thoughts?
