I've been chasing a Code 15 for three weeks. I've replaced the sensor and pig tail (they were shot) and did see an immediate improvement in idle but after just a short drive it resets. I'm at loss as to where to turn. I've read everthing in search and looked at you-tube for ideas. What say the brain trust here in technical. Thanks in advance for the help!

Engine cold read the resistance across the CTS sensor. Plug it back in, go to the ECM unplug the connectors and read the resistance there between the yellow wire of the CTS and the black return wire. Are they near the same? If not, why?

If I understand correctly; with the new CTS fitted and after a 30-minute drive, the ECM didn't clear the code?

Did you check the wiring connections at the ECM as phonedawgz suggested?

Code 15 - Coolant Temperature Sensor Circuit
http://www.fieros.de/en/v6help/code15.html

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 05-03-2025).]

quote Originally posted by Vintage-Nut: If I understand correctly; with the new CTS fitted and after a 30-minute drive, the ECM didn't clear the code? Did you check the wiring connections at the ECM as phonedawgz suggested? Code 15 - Coolant Temperature Sensor Circuit http://www.fieros.de/en/v6help/code15.html

Correct. New sensor, fitting and pigtail. Code reappeared after a short test drive. I have not checked the condition of the wiring at the ECM connection but I will do so later this evening. I will report back with what I find. Appreciate the help.

Only a Thought...

Years ago, brand-new parts were good quality and functioned out the box around 99.999%

But these days are gone, and members had been reporting more often that 'brand-new' parts means nothing and been found 'bad' from the box....

Check the cold resistance of the "new" sensor to see if it is approximately right or not....


IF it is 'close' - then compare the resistance at the sensor plug to the wiring at the ECM end...

Reporting back in with initial testing reports. ( I will retest again tomorrow to verify). Readings at the sensor and then pigtail at the ECM are 'close'. I'm guessing a new ECM based on the info in 'search' is in my future.

For your informational purposes only.....

I had 'weird problems' with a 1992 ECM in a different vehicle {not Fiero}, so I sent my computer to ECU Exchange for testing.

They found a bad power supply which created distortion on the reference voltages.

They repaired it; I got it back; installed the computer and the vehicle runs great now......

The Grand Total for the Repair?

Around $140 including shipping from Cailfornia to Florida

Merely my input,
VN

quote Originally posted by Vintage-Nut: For your informational purposes only..... I had 'weird problems' with a 1992 ECM in a different vehicle {not Fiero}, so I sent my computer to ECU Exchange for testing. They found a bad power supply which created distortion on the reference voltages. They repaired it; I got it back; installed the computer and the vehicle runs great now...... The Grand Total for the Repair? Around $140 including shipping from Cailfornia to Florida Merely my input, VN

Input greatly appreciated. I am going to order a Cardone unit from Rock Auto as none of the parts stores stock these unit (and they shouldn't). I was a Cardone distributer back in my auto parts days and I trust the brand. I'll post results when the unit arrives.
Thnaks again.

quote Originally posted by steve308: Reporting back in with initial testing reports. ( I will retest again tomorrow to verify). Readings at the sensor and then pigtail at the ECM are 'close'. I'm guessing a new ECM based on the info in 'search' is in my future.

If those resistances match then the wiring is okay. But is the resistance correct for the temperature? Check against the chart posted by vintage-nut.

What car do you have? I don't know if "code 15" is the same on every ECM.

quote Originally posted by armos: If those resistances match then the wiring is okay. But is the resistance correct for the temperature? Check against the chart posted by vintage-nut. What car do you have? I don't know if "code 15" is the same on every ECM.

Yes, checked against temperature. 86 -2.8 - Code 15 on Fiero appears to be consistant across the board.