E038

TEST CONDITIONS
1. Codes EO14 and EO15 not set;
2. Coolant temperature equal to or greater than -5°C.

FAILURE CONDITIONS
MAT less than or equal to -37°C

ACTION TAKEN
1. ECM turns on "Engine Control System Fault" telltale;
2. ECM commands BCM to display "Service Soon" message on Driver Information Center (DIC);
3. AIR management system is disabled;
4. ECM substitutes 40°C for MAT when coolant temperature equal to or greater than 40°C;
5. ECM substitutes coolant temp for MAT when coolant less than or equal to 40°C.

DESCRIPTION
The MAT sensor is a thermistor, or variable resistor, that varies resistance based on temperature. The MAT sensor is a "Two-wire" sensor with a reference, or signal, voltage coming from the ECM to sensor pin B (CKT 472), and a sensor reference ground on Pin A (CKT 476). As the temperature of the sensor increases, resistance decreases. As the resistance decreases, the signal voltage from the ECM decreases due to less voltage drop across the MAT sensor. High temperature will result in low signal voltage on CKT 472 and low temperature will result in high signal voltage on CKT 472.

The ECM uses MAT information to correct the fuel delivery rate for the temperature of the air that is entering the intake manifold.

Code EO38 sets when the ECM sees a low MAT reading when coolant is equal to or greater than -5°C.

Notes on Fault Tree
1. If the MAT sensor is open, ECM data ED05 should read -35°C or less. If not, then the sensor signal is not open at this time, see "Note on Intermittents."
2. Checking ECM and sensor circuitry from ECM to sensor connector. If ECM Data ED05 reads 148°C - 151°C with connector Pin A shorted to Pin B, then the sensor circuit and ECM are OK.
3. Checking for an open sensor ground.
4. Checking ECM's ability to recognize short to ground on MAT input.
5. Fault is most likely at ECM connector or ECM. Before ECM is replaced, perform "ECM Replacement Check", Chart C-1. Chart C-1 Vehicle Will Not Enter Diagnostics
6. The MAT sensor can be damaged by backfire in the intake manifold and by excessive intake heat due to valve train faults. If the vehicle has had multiple MAT sensor replacements, check for signs of backfire or high intake manifold air temperature due to improper valve train operation.

Notes on Intermittents
Manipulate CKT 472 and 476 wiring, MAT sensor connector, and ECM connector while observing ECM Data ED05. If the failure is induced, the Data value will jump from its normal value to the open circuit reading of -35° to -40°C. Disconnect and reconnect both the MAT sensor and ECM connectors and ensure that they are latched. If wiring and connectors check out ok, substitute a known good MAT sensor and retest.