DTC 34

Code 34 "Pressure Sensor Circuit":

Wiring Diagram - Pressure Sensor Circuit:

CODE 34

PRESSURE SENSOR CIRCUIT (SIGNAL VOLTAGE OUT OF RANGE)


Circuit Description:

Code 34 indicates that the ECM has monitored the following:

Pressure outside a specified value range (monitored by ECM as voltage at terminal "20").

Engine rpm less than a specified value.

Engine at operating temperature.

All the above for a time greater than specified.

The pressure sensor is a dual purpose MAP sensor. When the ignition switch is turned "ON," a barometric pressure signal is sent to the ECM prior to the engine cranking. From this baro reading, the ECM calculates the altitude at which the vehicle is being operated and selects the correct spark and fuel calibration table. The ECM also calculates altitude changes from the pressure sensor while the engine is running when throttle is near or at WOT.
The pressure sensor also serves as a vacuum sensor. The ECM uses this signal to adjust spark and fuel according to engine load. The ECM inverts the pressure sensor voltage and sends it on the ALDL as a vacuum sensor value. While reading the pressure sensor voltage with a "Scan" tool with ignition "ON," it will be close to 0 volt, and with the engine running at idle, it will be above 2.5 volts. If voltage is measured at the pressure sensor and/or the ECM with a voltmeter, the voltage will be close to 5 volts with ignition "ON" and less than 2.5 volts with the engine running at idle.


Test Description: Numbers below refer to circled numbers on the diagnostic chart. (With "Scan" tool.)

1. An engine that is able to maintain 18" of intake manifold vacuum will show about 4.0 volts on a scanner.

2. If ECM and wiring are OK, .58 volt will show on scanner.


3. Low vacuum to sensor will show as low voltage on "Scan" tool.

4. If CKT 452 is open, high voltage will show on "Scan" tool.

NOTE: Also be sure to check the TPS. A voltage reading of 2 volts or less at near wide open throttle (WOT) may set a code 34. See ADJUSTMENT PROCEDURES - TPS.