DTC 33

Chart - Code 33 Mass Air Flow Sensor (Signal Frequency High):

Wiring Diagram for Mass Air Flow Sensor.:

CODE 33 - MASS AIR FLOW (MAF) SENSOR CIRCUIT (GM/SEC HIGH)


Circuit Description:
The Mass Air Flow (MAF) Sensor measures the flow of air entering the engine. The sensor produces a frequency output between 32 and 150 hertz (3 gm/sec to 150 gm/sec.). A large quantity (high frequency) indicates acceleration, and a small quantity (low frequency) indicates deceleration, or idle. This information is used by the Electronic Control Module (ECM) for fuel control and is converted by a "Scan" tool to read out the air flow in grams per second. A normal reading is about 4-7 grams per second at idle and increases with rpm.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Code 33 will set if
Ignition "ON" and air flow exceeds 20 gms/sec.
OR
Engine is running less than 800 rpm.
Throttle Position Sensor (TPS) is 10% or less.
Air Flow greater than 150 grams per second (high frequency).
All of the above are met for 5 seconds or more.

Diagnostic Aids:

The "Scan" tool is not of much use in diagnosing this code, because, when the code sets, gm/sec will display a default value. However, it may be useful in comparing the signal of a problem vehicle with that of a known good running one.


Check For:
Poor Connection or Damaged Harness; Inspect ECM harness connectors for backed out terminal "B6", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Intermittent Test; If connections and harness check out OK, "Scan" MAF, while moving related connectors and wiring harness. If the failure is induced, the MAF display will change. This may help to isolate the location of the malfunction.

Mis-routed Harness; Inspect MAF Sensor harness to insure that it is not too close to high voltage wires, such as spark plug leads.

Miscalibrated MAF Sensor; The Mass Air Flow Sensor may be tested, for being out of specification, by using a MAF Sensor test tool J-36101, or equivalent. This tester will only indicate a sensor that is shifted completely from its calibrated frequency range.