The manifold absolute pressure (MAP) sensor responds to changes in the intake manifold pressure (vacuum). A 5 volt reference is applied to the sensor. A variable resistor moves in relation to manifold pressure and sends a voltage signal to the ECU on the 5 volt reference circuit. This voltage signal varies from 1.0-1.5 volts at closed throttle (high vacuum) to 4.0-4.5 volts at wide open throttle (low vacuum).

This is a type B DTC.

The ECU will illuminate the MIL and set DTC 33 after two consecutive driving cycles with the fault active.

The ECU will default to gasoline when one failure has occurred, even if it is intermittent. The vehicle will not attempt to run on CNG again until the next key cycle.

The ECU turns the MIL OFF after three consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.

Always diagnose ANY PCM codes that are present before beginning this diagnostic procedure.

With the ignition ON and the engine OFF, the manifold pressure is equal to the atmospheric pressure with the signal voltage high. The ECU uses this information as an indication of the altitude of the vehicle. Comparison of this reading with a known good vehicle using the same sensor is a good way to verify the accuracy of a suspect sensor. Readings should be the same, plus or minus 0.4 volt.

Refer to Intermittent Conditions .

The number(s) below refer to the step number(s) on the diagnostic table.

Important: The electrical connector must remain securely fastened.

Important: Disconnect the sensor from the bracket and twist the sensor by hand in order to inspect for intermittent connections. If the output changes greater than 0.1 volt, it indicates a bad connector or connection. If the output changes less than 0.1 volt, replace the sensor.

Important: After removing the MAP sensor from the intake manifold, replace the MAP sensor to intake manifold seal.

3. This step verifies that the high circuit voltage problem is still present.

4. With connector C018 disconnected and the voltage still present, this eliminates the ECU as the possible cause of the fault.