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#1 Old 12-08-2008, 12:05 PM
firecadet613
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P0128 Help

My D is throwing a P0128. I reset the computer and a few days later it set it again. What should I be looking at first? It has a new thermostat in it that I installed back in May when I flushed my cooling system completely and refilled with new Mopar HOAT fluid. It's a Stant 203 degree unit I believe.

Should I replace it again, or look at the temp sensor?

2006 Ram 2500 QC 4x4 Hemi
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#2 Old 12-08-2008, 02:52 PM
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What did the code say? Overheat?

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#3 Old 12-08-2008, 06:47 PM
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Sounds more like a sensor, what does your temp gage show? Are you over-heating as well?

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#4 Old 12-08-2008, 10:04 PM
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Sorry I should have put this in the original post. P0128 is the truck taking too long to warm up, at least thats what the PCM states. I have noticed since it's gotten colder out it does take a bit longer to get warm on the gauge. I did have a problem earlier this year where the gauge said it was running a bit on the warm side but after changing the coolant and thermostat it still said the same. Maybe its my temp sensor. Has anyone changed one yet?

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#5 Old 12-09-2008, 05:04 AM
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Could be a stuck open thermostat or a defective sensor.

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#6 Old 12-09-2008, 07:59 AM
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Thats what my guess is. What would you try first?

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#7 Old 12-09-2008, 11:59 AM
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Does it take a long/longer time for the heater to blow hot air?

If not - temp switch bad.
If so - open thermostat.

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#8 Old 12-09-2008, 12:22 PM
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I haven't noticed an increase in time. Temp switch it is. I picked one up this AM...looks like I have to move my a/c compressor to change it.

Anyone change it out before?

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#9 Old 12-09-2008, 08:36 PM
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Talk about a PIA to get to. Its burried under the A/C Compressor.

Got it changed out...I'll report back in a few days if it did the trick or not.

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#10 Old 12-09-2008, 08:48 PM
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#11 Old 12-10-2008, 11:09 AM
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going back to the Thermostat. When you put it in make sure the air release tab is up.

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#12 Old 12-10-2008, 11:28 AM
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Sorry, I'm usually more thorough with DTCs...here's some info.

P0128-THERMOSTAT RATIONALITY

Theory of Operation
The PCM predicts what the engine coolant temperature should be, based on the engine coolant temperature at start-up, ambient temperature and how the vehicle is subsequently driven. The predicted engine coolant temperature is compared to the Engine Coolant Temperature Sensor reading. The error between the two is calculated and integrated with respect to time. When the Thermostat diagnostic runs, the integrated error is compared to a calibrated threshold and pass/fail is determined. Separate pass and fail thresholds are used in order to improve accuracy of the diagnostic.

Oh and make sure your coolant isn't low.

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#13 Old 12-24-2008, 07:40 AM
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My wife drove my truck to work today. It spit back out the P0128. It has a brand new temp sensor, is the only other possible cause the thermostat? Coolant is about 6 months old and same with the thermostat.

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