Diagnostic Aids

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if signal and ground circuits become open.

Fig 1: Code 23, MAT Sensor Temp Lo Ckt Diag

Fig 2: Code 23, MAT Sensor Temp Lo Flow Chart