Rick's Free Auto Repair Advice

P0128 Coolant Thermostat

Common causes of P0128

A Po128 trouble code is generic OBDII trouble code and is defined as: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What is a P0128 code really?

When you start the engine, the ECM is watching the engine coolant temperature sensor to see how quickly the engine is warming up. Since it knows the engine temperature as you first turn the key to start and it knows the ambient temperture (from the intake air temperature sensor), it can do a quick calculation based on engine RPMS of how long it will take to reach operating temperature. If it doesn’t respond as quickly as expected, the ECM will set a P0128 trouble code.

The ECM can also set a P0128 if the engine reaches operating temperature but can’t maintain that temperature.

A P0128 can be caused by

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

Most common DIY mistakes when fixing a P0128

If you take your car to an auto parts store, there’s a 99.999%

ECT, engine coolant temperature sensor

Coolant temperature sensor

chance the clerk will tell you to replace the engine coolant temperature sensor. You’ll be thinking, “Yeah, cheap part and easy fix. So yeah, I’ll go for it.”

Guess what? In most cases you’ll have wasted time and money. The single most common cause of a P0128 is a faulty thermostat. Why? Because it’s a mechanical device and there’s no specific trouble code to tell you how it’s operating. The ECM has to infer based on the engine coolant temp sensor(DCT) and the intake air temp sensor (IAT).

Sure, the ECT and IAT can be out of spec, but if they’re that far out of spec, the ECM normally sets a trouble code for those sensors.

How to diagnose a P0128

Start by checking coolant level. If low, top off.

The easiest way to diagnose a P0128 is with an infra-red non-contact thermometer. Aim it at the thermostat housing and read the temperature. Start with a cold engine. The reading at the thermostat should be close to ambient temperature. Start the engine and watch the temperature rise. It will rise slowly at first, but should climb rapidly once it gets going. If you don’t see an increase in the rate of temperature rise, the thermostat is opening too quickly and circulating the coolant too soon. The fix is to replace the thermostat with a high quality unit—STANT or OEM only. I’m NOT a fan of Moto-Rad fail safe thermostats. Don’t ask me how many of those I’ve had to replace!.

If you do see a rapid rise, whip out your digital multimeter and backprobe the ECT. Compare the ECT reading with your thermometer readings. If they’re close, move on to check cooling fan operation.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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