PO 128 Coolant temp too low - FIXED - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > OEM Issues

OEM Issues Original Equipment Repairs and/or Problems

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  #1  
Old 12-08-2010, 10:33 PM
Gevans17 Gevans17 is offline
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2004 Chevy TrailBlazer LS
Dark Green Metallic 4.2L I6 2WD
Join Date: Nov 2006
Location: nawlins
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PO 128 Coolant temp too low - FIXED

2004 TB SES light came on. Had Autozone scan codes.... came back as "PO128 Coolant temp too low". Reading on temp gauge was 190 degrees, while it normally runs around 210. Engine running great though. Figured thermostat must be stuck open. Picked up a new thermostat and coolant temp sensor. Decided to change the temp sensor when i changed the thermostat, due to the terrible access.

However, when i removed the thermostat, it was closed. Changed it anyway along with the temp sensor. Worst part was getting the lower rear bolt on the thermostat housing back in. Had to remove the front wheel just to see where the hole in the thermostat housing lined up with the hole in the block. My hat is off to all of you (liars) who were able to change it without pulling the alternator!!

Put it all back together..no light, no leaks..just some minor bleeding. So now its fixed, I'm just not sure what caused it in the first place? Would a bad coolant temp sensor cause PCM commands that result in a lower engine operating temperature??
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  #2  
Old 12-09-2010, 07:04 PM
Chickenhawk's Avatar
Chickenhawk Chickenhawk is offline
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2004 Chevy TrailBlazer LT
Medium Red Metallic 4.2L I6 4X4
Join Date: Apr 2008
Location: Canada
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Defective thermostats get 'sticky' but they rarely stick and stay wide open.

The code could be because the engine was running colder than it should because the thermostat was defective OR it could be because the temp sensor caused the PCM to THINK it was running too cold. Either way, the result is the same. (Both the PCM and the gauge get their reading from the same temp sensor.)

In your case, it was likeliest to be the thermostat but it is always good to change both. The sensor is exposed to the coolant and this can cause progressively lower readings over the years. (Generally, if the reading fluctuates on the gauge, it is likely the sensor. If it is steady but always low, it is likely the thermostat.)

The temp gauge is not linear though. One small tick to the left is actually a pretty large drop in temperature, and it would have to be larger yet for the SES to light a code.

Good job, and you should see some improvements in mileage!
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  #3  
Old 12-10-2010, 07:26 PM
Gevans17 Gevans17 is offline
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2004 Chevy TrailBlazer LS
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Po 128

Thanks Chickenhawk. I didnt realize there was only 1 temp sensor for PCM and dash gauge. No way i was going to put it all back together without changing both T-Stat and sensor.
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