DTC 33
Code 33 - Mass Air Flow (MAF) Sensor Circuit:
Wiring Diagram for Mass Air Flow (MAF) Sensor:
CODE 33 - MASS AIR FLOW (MAF) SENSOR CIRCUIT (GM/SEC HIGH)
Circuit Description:
The Mass Air Flow (MAF) sensor measures the amount of air which passes through it. The Electronic Control Module (ECM) uses this information to determine engine load for fuel delivery control.
The oil pressure switch or the ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay which provides the 12 volts needed by the MAF sensor.
The ECM provides a current limiting 5 volts on the signal line (CKT 998). The MAF sensor then changes the signal by dropping the voltage so that with low air flow the ECM sees a low voltage and a high air flow will cause the ECM to see near the 5 volts supply.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
Code 33 indicates: ECM has seen flow in excess of 45 grams per second (above about 2.2 volts) for one second when:
^ Engine is first started
^ OR
^ Throttle Position Sensor (TPS) is less than 1/4 throttle and RPM is less than 2000.
Due to the 5 volts pull-up resistor in the ECM if CKT 998 becomes open, the ECM will see a high voltage signal and set a Code 33.
1. This test will determine if the conditions to set the code still exist.
2. With the Assembly Line Diagnostic Link (ALDL) terminal "G" jumpered to 12 volts, there should be 12 volts at the sensor. If no voltage is present, make sure that the fuel pump is running, If not, repair fuel pump circuit.
3. If a burn off signal is present at the MAF sensor with the engine running, a Code 33 will set. Be sure no voltage is present on CKT 994 for the first 2 seconds after the ignition is turned "ON" or after the 2 second period.
4. The ECM sources a voltage (4-6 volts) to the MAF sensor on CKT 998. This test checks for that voltage.
Diagnostic Aids:
Intermittent: By jumpering the fuel pump test terminal ("G" terminal of ALDL) to 12 volts, the MAF sensor will stay powered up and the signal line should see a low voltage, less than 250 mV or low grams per second on a "Scan" tool. By wiggling the related wiring the intermittent may be detected. Also, an erratic signal with the engine running may indicate faulty wiring or components.