5

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. – Duncan Rager Feb 11 '15 at 13:10
  • Duncan, nice job writing this question and providing prior research. – Lynn Crumbling Feb 11 '15 at 20:48
3

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. – Pᴀᴜʟsᴛᴇʀ2 Feb 11 '15 at 14:26
  • Just had to mention that I thought this was a really COMPLETE answer. +1 – Lynn Crumbling Feb 11 '15 at 20:47
  • Turned out to be the thermostat. Quick fix, thanks for the detailed response. – Duncan Rager Mar 19 '15 at 20:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.