Code 23A - Mat Circuit (Low Temperature Indicated): Notes

Manifold Air Temperature (MAT) sensor uses a thermistor to control signal to ECM. ECM applies 4-6 volts to circuit No. D7 of sensor. When manifold air is cold, sensor resistance is high and ECM will see a MAT signal. As air warms, sensor resistance is low and MAT signal drops.

NOTE: Test numbers refer to numbers on diagnostic chart.

Code 23A will set if engine has been running for longer than 3 minutes and signal voltage indicates a MAT temperature less than -40°F (-40°C).
This simulates conditions for Code 23B. If Scan tester reads high temperature, ECM and wiring are okay.
This checks continuity of circuits No. D2 and D7. If circuit No. D2 is open, Code 21A may also set.