Had the P0128 code pop up. Replaced thermostat, now the car warms up at idle with fan turned off, but when you either drive it or turn the heater fan on it drops the coolant temp down to around 102F

heater blows cold air idle chrysler 300 Questions & Answers ...

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As you have determined, the P0128 code is related to a low coolant temperature.

This can be one or two issues - the thermostat or the temperature sensor itself. If you're confident that the thermostat was properly replaced - you should probably look into the temperature senor itself.

Here's a video about the P0128 code diagnosing process.

First off who ever said $400 to change a thermostat is trying to rip you off. The actual part is like $20 bucks max and its so easy to change. I can guide you through it if you would like.

The P0128 code is Coolant Thermostat Malfunction.

These are the possible causes for this code:
- Insufficient warm up time.
- Low engine coolant level.
- Leaking or stuck open thermostat.
- Malfunctioning engine coolant temperature sensor.

These are the possible quick fixes:
- Top OFF Engine Coolant
- Replaced Thermostat

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

A code P0128 may mean that one or more of the following has happened:

• Low engine coolant level
• Leaking or stuck open thermostat
• Faulty cooling fan (running too much)
• Faulty coolant temperature (ECT) sensor
• Faulty intake air temperature (IAT) sensor

Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

• Verify coolant strength & level
• Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
• Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
• Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
• If the above items check out good, replace the thermostat

REFERENCE: http://www.obd-codes.com/p0128

I hope this helps a bit!

P0128 indicates that the car doesn't "think" it is warming up quickly enough. So either it really isn't getting warm enough, or the sensor indicating the temperature is bad. If it isn't getting warm enough the causes include a thermostat stuck open, low coolant, and a cooling fan that is running when it shouldn't be.

For a good explanation and more insight visit
http://www.obd-codes.com/p0128

Keep in mind that many cars have two coolant temp sensors. One for the computer, and one for the temp gauge. Make sure you are working with the correct one.

P0300 is a code for multiple misfire. Which could be created to the P0128
P0128 PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat.
A code P0128 may mean that one or more of the following has happened:

• Low engine coolant level
• Leaking or stuck open thermostat
• Faulty cooling fan (running too much)
• Faulty coolant temperature (ECT) sensor
• Faulty intake air temperature (IAT) sensor

Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

• Verify coolant strength & level
• Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
• Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
• Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
• If the above items check out good, replace the thermostat

Good luck and hope this helps. keep me posted, be glad to know your car is running 100% again soon.

1. Check for a code P0128 stored in the Powertrain Control Module (PCM)

2. If there is a P0128, clear the code and see that the coolant fan turns off.

3. Replace the thermostat and check that the coolant temperature stays close to 195 degrees while driving.

PO172 means that an oxygen sensor in bank 1 detected a rich condition (too little oxygen in the exhaust). I would replace the thermostat first. The computer could be injecting too much fuel because it thinks it never gets to operating temperature. That may keep the light off.

2000 jetta warms up when idling, but gets cold as soon as I start driving.

Please use this link to replace the thermostat on your car:

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528003fae2


Good luck...