I did the plenum fix about 3 weeks ago, replace'd everything and put in a 180 degree thermostat etc. its running great towed my trailer to the beach over 100 mi each way with no problems. came home from work today and the check engine light came on ran the sct and said the thermostat is inactive. anyone knwo why??

 

where does your temp gauge hang out at? It is possible that the PCM is seeing the low temp, and never getting up to the 195-200 degrees it expects..... so, it interprets that as a failed thermostat.

 

+1 The newer trucks will do that.

 

yeah it is running a lot colder then normal, it used to run around 180-190 ish a little below the 210 line now it runs halfway between 130 and the 210 line or lower.

 

Need to reflash the PCM, so it knows about the lower temp thermostat would be my guess.

 

or the temp sending unit going bad