I recently failed an emissions test because my CAT and EVAP readiness monitors were "Not Ready" following a reset check engine light a few weeks back, which told me that the engine coolant wasn't running hot enough. I've confirmed this with a ODB-II scan tool.

According to pg 13 of this document (which is for a 2002, but I assume they are similar), my engine coolant temperature needs to be 176 degrees F for the CAT monitor to start running. My scanner is telling me that my ECT is running at ~160 degrees F.

I can't figure out if this means a sensor is busted, or something else (like cold weather) is causing the ECT to be that low. Can anyone recommend a next step that I might try on my own instead of taking it into the shop?

  • How cold is the weather where you are? That might be important information
    – Zaid
    Feb 11 '15 at 4:42
  • Lately it's been in the 30s and 40s. Feb 11 '15 at 13:10
  • Duncan, nice job writing this question and providing prior research. Feb 11 '15 at 20:48

In terms of possible root causes for failure, the following scenarios are likely culprits for a cooling system to run too cool:

  • A temperature sensor that's reporting lower-than-actual temperatures due to issues like drift/fouling/calibration.

  • A sticking thermostat that is flowing more coolant than necessary to maintain 176 °F.

  • Really cold ambient air temperature which prevents the system from settling down at the desired 176 °F.

The most likely culprit based on the information provided is a sticking thermostat:

  • If the temperature sensor malfunctions, one would observe negative temperatures with the scan tool, which in this case isn't happening. While temperature misrepresentation is a possibility, most temperature sensors have a tendency to either work well or not work at all.

  • A stuck-open thermostat will give slightly-below-par engine coolant temperatures, as is the case here.

  • More information is needed about the ambient air temperature in order to comment on it.

  • I would concur with the sticking thermostat. I doubt ambient air temperature would have much of anything to do with it. As long as the thermostat is functioning correctly, there shouldn't be an under temperature condition. A gasoline engine produces way more heat than it uses. Once up to temperature it will maintain it without issue. A completely closed thermostat will allow that to happen fairly quickly. Feb 11 '15 at 14:26
  • Just had to mention that I thought this was a really COMPLETE answer. +1 Feb 11 '15 at 20:47
  • Turned out to be the thermostat. Quick fix, thanks for the detailed response. Mar 19 '15 at 20:01

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