Hi PFF!
This is my first post so I am sorry if it may be incomplete in any way. please feel free to let me know.
that said...
I just recently started getting a code 15 (Coolant temperature sensor signal voltage too high) on my 85 GT.
I have searched the forums and found one or two posts directly relating to code 15 but none come to a conclusion. (I admit I may not have searched enough).
I have checked and the ECM is sending out 5 volts. code 15 sets if the ECM receives 5 volts for too long. the CTS shouldn't allow that to happen unless it gets REALLY cold
I have also disconnected the CTS pigtail, cleared codes and ran the engine for about a minute. code 15 returns.
this led me to believe the CTS itself is not causing the issue.
reading through the 85 factory service manual, I noticed something.
The CTS and the TPS share a common ground at pin D12 on the ECM.
Both the CTS and TPS get a seperate 5 volt signal from the ECM.
is it possible for the TPS to short out, sending 5 volts down the common D12 wire making the computer set code 15?

Sorry for the lengthy post (it seems long to me).

-Rich

I had somehing similar happen: The car would suddenly start running like crap, and the check engine light would come one, codes 15 and 21 (CTS and TPS) and the car would die and not restart. Replaced both parts, still happened. Couldn't figure it out, took it to a mechanic who spent 3 hours trying to figure it out. I looked at the manual, and found that common ground wire between the two sensors. Replaced the ground wire, car's been fine since. Second time I fixed something the OBDII-spoiled mechanics couldn't figure out

Also make sure you unpluged the correct sensor; there's one for the computer, one for the gauge, and one for the fan.

And welcome to the forum!

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1984 Fiero SE

[This message has been edited by RWDPLZ (edited 02-26-2009).]

Thank you RWD, very much appreciated.
I definately have the right sensor. horizontally threaded near the center of the lower intake manifold on the passenger side of the car.
Being that the car is ~24 years old, would it be a good idea to remove the harness and go over it, possibly replacing certain wires? if so how would I go about removing it safely and how much of each wire should I replace (I'm thinking everything up to the bulkhead connector, the remaining wires inside the passenger compartment should be good).

I seem to have resolved my code 15...
I sourced out another computer to do some testing with, (computer from unknown year v6 w/ 5speed.)

The "junkyard" computer when plugged into my car would not run the fuel pump which perplexed and angered me so I removed the calpak and prom and swapped them between both computers.

Now, My Fiero is happily running well again with the original computer and NO codes whatsoever!

on a side note, we also replaced the rear exhaust manifold as mine was leaking badly, and the IAC (we lost the spring but we definately cleaned it really well before we broke it)