2009 Aveo5 P0128, temp gauge and A/C not working

[SilentMatt]

My car has code P0128 Coolant Temperature Below Thermostat Regulating Temperature and temp gauge
and A/C don't work. I've read that this is a an issue with the thermostat sticking open, but wouldn't my temp gauge go up a little? It stays in the rest position all the time. My heater blows pretty hot as well.

Any input is greatly appreciated

[EAD15]

Here's the code explanation from the service manual. Maybe it will give you an idea where to start.

"The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the closed loop temperature or to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the closed loop temperature, diagnostics that use the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the closed loop temperature or the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, the applicable DTC sets."

Also here is the diagnostic steps from the manual:

Circuit/System Verification

• Perform the Loss of Coolant to verify correct fluid level.
• Perform the Thermostat Diagnosis to verify correct normal operating temperature.
• Engine idling, observe the DTC information with a scan tool. DTC P0128 should not set.
• Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

• Ignition OFF, disconnect the harness connector at the ECT sensor.
• Ignition OFF, test for less than 5 ohms between the low reference circuit terminal A and ground.
  
  • If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
  
  
• Ignition ON, verify the scan tool ECT sensor parameter is at -40°C (-40°F).
  
  • If greater than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.
  
  
• Ignition ON, clear the DTC information with a scan tool.
  
  [TD="class: clsImportantLabel"]IMPORTANT:[/TD]
  
  
  

  If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the sensor may be damaged.

  
  
• Install a 1A fused jumper wire between the signal circuit terminal B and ground. Verify the scan tool ECT sensor parameter is greater than 128°C (262°F).
  
  • If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  
  
• If all circuits test normal, test or replace the ECT sensor.

Component Testing

• Ignition OFF, disconnect the harness connector at the ECT sensor.
  
  [TD="class: clsImportantLabel"]IMPORTANT:[/TD]
  
  
  

  A thermometer can be used to test the sensor off the vehicle.

  
  
• Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the table and verify that the resistance is within 5 percent of the specification.
  
  • If not within the specified range, replace the ECT sensor.
  
  

[SilentMatt]

Thank you

I just pulled the battery terminals to clear the code and now the gauge and A/C work.
It gets just about to a quarter of the way up while driving and just a little hotter while
sitting still.

I think my thermostat is indeed stuck open. I'll do a little more trouble shooting before I order one though.

[SilentMatt]

I ended up buying a $20 thermostat off ebay to see if it was half decent. I almost went with one from rockauto, but I took the gamble for a third of the price. So far I have about 25 miles on it and my temp stays at just under halfway once it's warmed up. When I pulled the old one, I expected it to be stuck open, but it wasn't. The only thing I saw was that the rubber seal around the actual valve portion was cracked and missing a section. I didn't test it to see want temperature it was opening at.

So far, so good. I'll keep you guys updated on how well it works.

[SilentMatt]

I have over 500 miles on the $20 t-stat and so far, so good. Now my downstream o2 sensor is out...

[SilentMatt]

Around 6,000 miles on the ebay t-stat, still working.