For 1990-2009 cars

# CODE 33 MASS AIR FLOW (MAF) SENSOR VOLTAGE HIGH DIAGNOSTIC

CODE 33 MASS AIR FLOW (MAF) SENSOR VOLTAGE HIGH DIAGNOSTICS

#### VEHICLES AFFECTED: 1987-86 F AND Y CARS W/5.0L AND 5.7L ENGINE ----------------- Some vehicles may experience a check engine light with a Code 33 stored which cannot be easily diagnosed. To promote better diagnosis of this condition, a brief explanation of the criteria for setting a Code 33 and an explanation of the possible causes follows.

Before beginning diagnosis, all 1986 models should be updated to the appropriate service PROM listed in bulletins: 86-657-106 and 86-650-109.

Mass Air Flow (MAF) power and burnoff relay part numbers should be, checked to assure that they agree with those listed in bulletin 87-650-119 and 87- 657-113.

Code 33 is set when the voltage at Pin B12 of the ECM exceeds 2.2 volts for 1 second or more either at startup or when the Throttle Position Sensor (TPS) is less than 20% and the RPM is less than 2000 RPM.

The ECM provides a 5 volt signal at pin B12 through a pull up resistor. This voltage must be brought below 2.2 volts by the MAF sensor circuitry during the conditions mentioned in the above paragraph or a Code 33 will set.

A condition whereby the MAf sensor circuit can not accomplish this can be caused in the following manners:

- An open or intermittent in Circuit 998 (dark green wire from ECM pin B12 to MAF sensor pin C)

- A condition causing voltage at MAF sensor pin D during crank or run such as:

- A short to ground in Circuit 900 (black wire from ECM pin D12 to MAF burnoff relay pin F).

- A faulty or incorrect burnoff relay

- A short to voltage in Circuit 994 (dark blue wire from pin A of the burnoff relay to pin D of the MAF sensor)

- A condition causing reduced voltage supply to pin E at the MAF sensor such as:

- An open, intermittent, or short to ground in Circuit 993 (red wire between pin E on the power relay and pin E on the MAF sensor)

- A faulty or incorrect power relay

- A faulty fuel pump relay

- Poor or intermittent ground on Circuit 450 (blk/white ground)

- Circuits 340 or 120 intermittent, grounded, open or spliced into with anything which draws enough current to lower the voltage by 2 volts or more for a 1 second period. This includes but is not limited to CB radios and alarm systems. (Circuit 340 orange extends from ECM pin Bl to MAF power relay pin A and fuel pump relay pin E; Circuit 120 tan/white extends from fuel pump relay pin A to MAF power relay pin D)

- An intermittent or low TPS switch causing the ECM to run a Code 33 check at a higher throttle angle (and therefore higher airflow rate) than specified.

- A faulty or poorly connected MAF sensor.

As evidenced by the above list of possible causes of Code 33, this circuitry is very sensitive to minor wiring discrepancies and poor or loose connections. All connections should be checked carefully. One method is to wiggle the connections while meeting the conditions for running a Code 33 test as listed earlier in this bulletin. In addition, they should be inspected for corrosion and/or bent pins.

This bulletin is extended to provide supplemental Code 33 diagnostic information and is not intended to replace the Code 33 diagnostic chart in the Service manual.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.