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Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind of annoying. That’s why Car Bibles did it for you! You’re welcome, now let’s get into solving your issue.

OBD2 P0128: What It Means

This code reads as “coolant temperature below thermostat regulating temperature.” All that means is that the coolant temp is too low; the car can’t get into its normal operating temperature. This prevents the car from going into the correct fueling mode, and means that it hurts emissions performance.

Likely Symptoms

Here’s what you may experience due to the P0128 code:

  • Check Engine Light.
  • Bad fuel economy. The car will be firehosing fuel because it thinks it’s cold.
  • High idle. The car won’t be able to warm up fully, so it stays in cold-start high idle mode.
  • Low coolant/water temperature displayed on gauge, if there is a gauge. Most cars have a “standard” point where the gauge sits; that’s operating temperature. If it’s lower than usual, then your temp is low.

Probable Causes

Here’s what could be causing the issue:

  • Bad thermostat. Sometimes the thermostat that regulates coolant temperature, can fail stuck open, letting too much coolant circulate and keeping the car too cold.
  • Missing thermostat. If you just bought a car and are unfamiliar with its service history, it’s worth figuring out if the car has a thermostat at all.
  • Bad coolant temperature sensor. Depending on the car, the coolant temp sensor can fail by no longer reading temperature, or fails by sticking at one temperature. Easiest way to verify is to see if the temperature gauge acts normally.

What Part Is Potentially Affected?

Your best bet will be a stuck open thermostat or a bad coolant temperature sensor. Beware, some cars have thermostats that fail closed, which will be clear when the car overheats. In that case, take a good look at the whole cooling system.

Possible Fixes

Here are the most common fixes to remedy the P0128 code:

  • Replace coolant thermostat. If the thermostat is stuck open or missing, that is likely your issue.
  • Replace coolant temperature sensor. The sensor can fail and show incorrect temperatures, some cars have multiple sensors so be sure to know which one to change. Usually, cars equipped with multiple sensors have a corresponding code for their failure.
  • Check for damaged wiring to and from coolant temperature sensor. In the rarest of cases, the wiring gets damaged and needs a repair. Highly unlikely, and at the very bottom of the list of possibilities. 

Finding The Parts You Need

Now that you’ve figured out what’s wrong with your hooptie, let’s talk about where you’re gonna find that part’s replacement.

There are plenty of places you can buy auto parts from, but Car Bibles gets paid if you click this Advance Auto link so that’s the one we’re serving up. Advance Auto Parts also has delivery, curbside pickup, and a host of helpful diagrams to aid your repair!

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