Do these ever go bad? Maybe lose their effectiveness, like false readings off enough to cause maybe a faster idle until heat soaked then settle down?

Give me some sound advice, then I'll disclose the rest of the story. Not trying to trick anyone.

 

I think it's pretty unusual for the stock EFI temp sensor to go bad, it's a pretty simple device.. Easy to test though, it's on sliding resistance / temp scale. I believe the exact measurements are in the FSM.. If you want to get fully diagnostic on it - test it in ice water, at room temp, and then in boiling water.

I have the values somewhere if you need them.
They do get heat soaked by design.. temp of the head / manifold / water shouldn't be that far off.

 

I'd think after 17 years it's certainly posssible.

 

I had a 92 Range Rover , when the coolant temp sensor crapped out it would cause very poor running , some times not start , and hot start problems. I replaced it and it ran smooth , I would have never thought it that would have made such a difference. I think it can cause the ECU to read and adjust wrong.

 

your definetly right.. on sum vehicles the coolant temp effects the ecu dramatically

 

Yeah the ecu will go in and out of open and closed loop depending on the temps and if it got stuck in say open loop cold start mode that would really cause some problems running right.