DTC P1258 – How To Replace your GMC Sierra ECT

DTC P1258 – How To Replace your GMC Sierra ECT


The GM specific OBDII trouble code of P1258 involves your Engine Coolant Temperature sensor. The specific wording behind this trouble code is Engine Coolant Overtemperature – Protection Mode Active. Put simply, your GM engine is overheating and in order for the ECU to protect the engine, DTC P1258 triggers limp mode. Limp mode is designed to limit ignition and fuel, while literally allowing you to limp your vehicle home or to the nearest mechanic.

When temperatures rise in your GEN III engine, your GM PCM monitors your Engine Coolant Temperature Sender (ECT). When the engine temperatures surpass 132º C for more than 10 seconds, the P1258 OBDII trouble code is triggered. Your On Board Diagnostic (OBD) system will light your Malfunction Indicator Lamp (MIL) to notify you of the problem.

If you are having overheating problems in your 4.8 liter GM engine, check our guide on how to prevent overheating in a GEN III engine, or our guide on troubleshooting overheating problems.


Today we’ll be showing you how to fix the OBDII trouble code of P1258 in a 2000 GMC Sierra with a 4.8 liter engine in it. The VIN ID of this particular truck is V, make sure you are working with the same series of 4.8 liter Vortec engine. When working on our how to replace your GMC Sierra ECT Guide, make sure you double check your VIN numbers, as there are many different versions of this engine.


The Engine Coolant Temperature sensor in your GMC is a two pin unit that’s located on the driver side of your engine. If you have already gone through the proper troubleshooting steps, and think your ECT is causing the P1258, it’s easy to replace as you’ll see.

Symptoms of OBDII P1258


  • Vehicle is in limp mode
  • White smoke from tailpipe
  • temperature meter in the red
  • Check Engine Light On

This guide on how to replace your GMC Sierra ECT assumes you have already done the proper legwork to make sure your van or truck is not really overheating. Check our guide here to do this now if you haven’t taken those steps.

Why am I replacing my ECT?


Your Sierra ECU stores the information regarding your 4.8 liter overheating when it first occurs. The ECU will store this information in your PCM Failure Records.

If you do not correct the overheating issue, or the sensor issue, the next time you start the car and the failure happens, your PCM updates the condition to the stored freeze frames. This DIY guide assumes you have already eliminated every other source of your overheating issues, and your ECT is just not acting right.

If you do not have a scan tool, GM calls for three consecutive driving sessions without a failure before the stored P1258 is erased or cleared.


Before getting started, you will need to disconnect or unplug your mass air flow sensor, and remove the MAF housing and air box. To do this, pull on the gray lock tab before depressing the five or three pin connector from your MAF.

With the MAF unplugged, go ahead and remove the bolts that hold the MAF housing and airbox in place. Remove the air box and intake tract, while leaving the fan shroud and coolant overflow tanks if possible.


It’s going to be a tight fit, but learning how to replace your GMC Sierra ECT usually does not involve removing the rather large and cumbersome fan shroud on your 4.8 liter GMC.

Now take a peek down at your engine and coolant sensor for your engine. If the headers are warm, take a break now and enjoy your favorite beverage.


This sensor is connected by way of your GMC Sierra wiring harness. Lift of the two prong ECT connector and disconnect.


Remove the old coolant sensor carefully, making sure not to burn yourself on the exhaust. When installing your new GM ECT, make sure to apply a layer of teflon tape to prevent any coolant leaks.

You now know how to replace your GMC Sierra ECT to resolve the OBDII DTC of P1258. Use a scan tool to clear your check engine light, or simply perform the three driving cycles as described above to fix your code.

Have any questions about our how to replace your GMC Sierra ECT DIY Guide? Leave them for us below!