Without Speed Density Update

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

Wiring Diagram for MAT & MAF Sensors.:

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


Circuit Description:
The MAF sensor measures the now of air entering the engine. The sensor produces a frequency output between 32 and 150 hertz (3gm/sec to 150gm/sec.), A large quantity (high frequency) indicates acceleration, and a small quantity (low frequency) indicates deceleration or idle. This information is used by the 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. The MAF sensor is powered up by a 7.5 amp fuse which is located underhood in the right side relay center and the sensor should have power supplied to it anytime the ignition is "ON." The MAF and MAT sensor are combined into one assembly located in the air duct.


Test Description:Numbers below refer to circled numbers on the diagnostic chart. Code 33 will set if Ignition "ON," engine "OFF," and air now exceeds 20 gm/sec. OR
Engine is running at less than 1300 rpm.
TPS is 8% or less.
Air now greater than 20 gm/sec. (high frequency).
The above three conditions are met for 2 seconds.


Diagnostic Aids:

The "Scan" tool is not of much use in diagnosing this code because when the code sets, the value displayed will be the default value. However, it may be useful in comparing the signal of a problem vehicle with that of a known good running one. Refer to "Intermittents" in "DIAGNOSIS BY SYMPTOM - NO TROUBLE CODE STORED". Inspect wire routing of high voltage wires such as spark plug wires. Such wires routed too closely to MAF wiring harness could possibly cause an intermittent Code 33.


*** NOTE: Before replacing MAF sensor, see TSB #886E31 DATED JUNE, 1990