Q: Check engine light on and the P0128 code is showing

asked by on

My car has the check engine light on. There is a P0128 code, which shows that the thermostat doesn't work correctly. The PCM shows that the engine isn't at the right temperature that it should reach from the startup. The gauge goes from cold to normal temperature in a few minutes from the startup. Is this an issue with the thermostat or something else?

The P0128 code is most likely caused by one of the following three issues: a faulty thermostat, a faulty engine coolant temperature sensor, or a processing error in the ECM. The coolant temperature needs to measured using a temperature gun from a cold start and compared to what the ECM is seeing all while the temperature sensor voltage drop is being monitored. This will locate the cause of the issue. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the necessary equipment and training to conduct a thorough diagnosis for the cause of the code, and then have the correct repairs made.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Check engine light on and the P0128 code is showing

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Fuel gauge stopped working - 1972 Pontiac Grand LeMans

Hello there, thanks for writing in. A faulty sending unit can be problematic for your 1972 Pontiac Grand LeMans. When the fuel gauge is not functional, there are 3 suspected components: the sending unit, wiring for the gauge and sending...

Q: Q: Code P0128

The code has to do with the thermostat not performing as specified, or the temperature sensor is not working correctly. You could first replace the thermostat, since it is the most common cause, and see if the code returns. If...

Q: My temperature gauge rises then goes back down.

This can be caused by a number of things such as low coolant levels, a faulty thermostat, or a failing coolant fan switch. As you may know, the coolant fan switch helps to maintain the proper coolant temperature by turning...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...