What causes this to read -40*? broken wire?

I replaced the IAT this morning and it still reads -40, I believe this is the lowest it will read. Car runs fine, just want it to read accurately.

 

Gotta be either a broken wire like you said... or a short in the sensor/wiring.

 

That temperature corresponds to "infinite" resistance = open circuit. Wire is broken, harness connector pin broken, etc. A short would show up a low resistance and indicate a high temperature.

 

How would the PCM respond to the -40 reading? Would it treat it as real and accurate, and adjust fuel/spark to correspond, or would the PCM "know" that it's a false reading?

 

Thanks fellas, just wanted to confirm before I tore into the wiring.

 

I'm not sure how the newer computers respond, but my father's 93 truck had a temp sensor go bad and do the same thing, it was super hard to get it started, and it ran like crap. $17.99 later it was fixed.

 

The PCM needs the coolant temp sensor to set the cold start A/F enrichment. Also needs it to determine when to go into closed loop A/F ratio control. The -40degF temp should set a code and turn on the SES light. When that happens, I suspect the PCM defaults to some sort of protective conditions, to prevent damage to the engine in the event the temperature is actually higher.

 

Good info!

 

I had this same problem, ended up being a splice came apart in my IAT harness. SES light on, and fans would come on and stay on once the engine got up to operating temp.

 

Got 'er fixed up, a wire had broken. Before I fixed it I noticed it idled a little wierd when I started it up in the morning. Also most of the day yesterday, it would not drop down to my normal 850rpm idle, tried to idle around 1100 instead. Also, didn't get an SES light 'till late last night after driving all day with it reading -40. Fixed now, Thanks fellas.