Powertrain Controls - ECM/PCM

Code 23 Diagnostic Chart:

TPS, CTS, & IAT Sensors (Circuit Diagram):

CIRCUIT DESCRIPTION:
The IAT sensor is a thermistor with a negative temperature coefficient (as temperature increases, resistance decreases. The PCM applies a 5 volt reference voltage through a resistor, to the sensor, on CKT 472, and then monitors the voltage in the circuit. When the air is cold, the sensors resistance is high, and the circuit voltage remains high (near reference voltage.) If the air is warm, sensor resistance decreases, causing drop in the measured voltage in the circuit. The PCM measures the voltage and calculates the temperature of the incoming air.

Code 23 will set if signal voltage indicates a intake air temperature below -34 C (-29 F) for 4 seconds. Due to the conditions necessary to set a Code 23, the "SERVICE ENGINE SOON" light will only stay "ON" while the fault is present.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. A code 23 will set, due to an open sensor, wire, or connection. An open circuit would cause the scan tool to display approximately -40 C (-40 F). This test determines if the wiring and PCM are OK.

DIAGNOSTIC AIDS:
^ Poor Connection or Damaged Harness. Inspect PCM harness connectors for backed out terminal "BA7," "BB7," improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent Test. If connections and harness check OK, monitor circuit voltage with "SCAN" tool, while moving related connectors and wiring harness. If the failure is induced, the display will change. This may help to isolate the location of the malfunction.