I am no expert in this area but the service manual covers this code pretty well. Inputs for setting code 15 come from coolant temperature (it is looking for low temp at start), and time since start. It goes on to say that coolant temp is one of the inputs used to control fuel delivery, engine timing, idle and converter clutch (TCC). I do know that incorrect timing can cause an engine to overheat, so that is a possible reason. The manual says to trouble shoot this way:
1) Disconnect coolant sensor, ingnition on and engine stopped.
2) Check voltage between harness connector terminals ckt 410 and 452
3) If you have 4 volts check across the coolant sensor terminals resistance should be less than 1000 ohms {at operating temp, on a cold engine less than 7,500 ohms}. If it is greater replace the sensor. If it is less thenlook for intermittent fualt in sensor circuit or connector.
4) If you have less than 4 volts, check for open in 410, 452. If open you have a faulty ECM connector or ECM.
This is from the troubleshooting flow chart with some interpretation of my own. I hope it helps.
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