What Does Your P0128 Code Really Mean?

If you aren't an automotive technician, then your car's error codes may be a bit of a mystery. Your car's OBD-II system monitors various sensors to detect signs of trouble, triggering one of these codes when something looks amiss. While OBD-II codes are beneficial for diagnosing problems, they can also be confusing to the untrained.

P0128 is a fairly common code that indicates a problem with your car's coolant temperature. Your motor operates within a relatively tight temperature range, and it will complain if it's running either too hot or too cold. In the case of P0128, your engine isn't reaching operating temperature quickly enough, and so the computer raises a red flag by turning on your check engine light.

Why Is a Cold Engine a Bad Thing?

You've probably heard about the dangers of overheating, but what about underheating? If your engine temperature remains below its intended level, you can expect it to operate less efficiently and wear out more quickly. Remember that your oil flows and lubricates better at warm temperatures, so a cold engine experiences significantly more internal friction.

If your engine is running too cold, you'll probably notice reduced fuel economy as the primary symptom. Under normal conditions, your car uses a fuel-rich mixture as it warms up, eventually arriving at a standard air-to-fuel ratio at nominal temperature. If your engine never gets warm enough, you'll remain in a fuel-rich regime that wastes gas and may cause damage to exhaust components.

What Is P0128 Trying to Tell You?

A common misconception with OBD-II error codes is that they point you directly at the failing component. In reality, your check engine light comes on when a sensor reads data that's out of spec. The P0128 error code isn't reporting a specific problem but rather a symptom: your engine is too cold. Finding the problem usually requires more investigation.

Stuck thermostats are one common cause of P0128 check engine codes. A thermostat that's stuck closed can rapidly destroy an engine, so most thermostats remain in an open position when they fail. The result is that coolant continuously cycles through your engine, preventing it from ever reaching its target operating temperature.

However, thermostats aren't the only potential culprit. You may also have a faulty sensor that's producing an incorrect reading, or there may be issues with your radiator fan or even the coolant itself. If you aren't confident that you know the underlying issue, then it's best to bring your car to an experienced mechanic to fix the actual cause and avoid needlessly throwing parts at the problem.

For more information on check engine light repairs, contact a local auto service.