Rick's Free Auto Repair Advice

P0128 Most Common Causes

P0128 most common causes and fix

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

What does a P0128 mean

The two most common causes of a P0128 are bad thermostat or a bad engine coolant temperature sensor. The vast majority of the time, it’s a bad thermostat that opens too quickly.

How the ECM sets a P0128 code

The ECM knows the engine coolant temperature. After you start the engine, it knows the RPMs and fuel mixtures. Based on those three readings, it expects the engine to reach a certain temperature within a set time window. If your engine doesn’t reach that temperature within the time window, the -ECM will set a P0128 trouble code.

The thermostat is supposed to stay closed at cold startup

At cold startup, the thermostat should be closed. Then as the engine heats up, it should slowly open. If it opens too quickly, the engine won’t reach the expected temperature within the time window.

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

That could be caused by a stuck open thermostat
Low engine coolant level
Small leak in the cooling system that prevents it from building full pressure
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.”

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 back probe the ECT. Compare the ECT reading with your thermometer readings. If they’re close, move on to check the cooling fan operation.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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