Accepted Answer

The 23 is:

Circuit Description:
The The Intake Air Temperature (IAT) sensor is a thermistor that controls the signal voltage to the PCM. The PCM applies a voltage (4-6 volts) on CKT 472 to the sensor. When the air is cold, the sensor (thermistor) resistance is high, therefore, the PCM will see a high signal voltage. If the air is warm, the sensor resistance is low, therefore, the PCM will see a low voltage.

Test DesIf the MAP sensor fails, the control module will substitute a fixed MAP value and use the Throttle Position (TP) Sensor to control fuel delivery.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step determines if Code 34 is the result of a hard failure or an intermittent condition. A Code 34 will set when:

• • When engine is less than 1200 rpm.
  • Manifold pressure reading less than 12.6 kPa, conditions met for 1 second. OR
  • Engine speed is greater than 1200 rpm.
  • Throttle angle over 21%.
  • Manifold pressure less than 12.6 kPa, conditions met for 1 second.

1. Jumpering harness terminals "B" to "C" (5 volts to signal circuit) will determine if the sensor is at fault, or if there is a problem with the PCM or wiring.
2. The Tech 1 "Scan" tool may not display 5 volts. The important thing is that the PCM recognized the voltage as more than 4 volts, indicating that the PCM and CKT 432 are OK.

Diagnostic Aids:
An intermittent open in CKT 432 or CKT 474 will result in a Code 34.

With the ignition "ON" and the engine "OFF," the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the PCM as an indication of vehicle altitude.
Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Reading should be the same + 0.4 volt.
Also, to test the MAP sensor, see Manifold Absolute Pressure Output Check or Intermittents.
See: Symptom Related Diagnostic Procedures\Intermittent Condition

Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than 0.1 volt indicates a bad connector or connection. If OK, replace sensor.

cription:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step will determine if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will set if all conditions are met:

• • A signal voltage indicates a Intake Air Temperature (IAT) below -30°C (-22°F) for 25 seconds.
  • Time since engine start is 4 minutes or longer.
  • No VSS (vehicle not moving)

1. A Code 23 will set, due to an open sensor, wire, or connection. This test will determine if the wiring and PCM are OK.
2. This will determine if the signal CKT 472 or the 5 volt return CKT 455 is open.

Diagnostic Aids:
A Tech 1 "Scan" tool reads temperature of the air entering the engine and should read close to ambient air temperature when engine is cold, and rises as underhood temperature increases.

Carefully check harness and connections for possible open CKT 472 or 455. Refer to "Driveablity Symptoms," for "Intermittents"

The "Temperature to Resistance Value" scale may be used to test the IAT sensor at various temperature levels to evaluate the possibility of a "skewed" (mis-scaled) sensor. A "skewed" sensor could result in poor driveability complaints

The 34 is:

.If the MAP sensor fails, the control module will substitute a fixed MAP value and use the Throttle Position (TP) Sensor to control fuel delivery.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step determines if Code 34 is the result of a hard failure or an intermittent condition. A Code 34 will set when:

• • When engine is less than 1200 rpm.
  • Manifold pressure reading less than 12.6 kPa, conditions met for 1 second. OR
  • Engine speed is greater than 1200 rpm.
  • Throttle angle over 21%.
  • Manifold pressure less than 12.6 kPa, conditions met for 1 second.

1. Jumpering harness terminals "B" to "C" (5 volts to signal circuit) will determine if the sensor is at fault, or if there is a problem with the PCM or wiring.
2. The Tech 1 "Scan" tool may not display 5 volts. The important thing is that the PCM recognized the voltage as more than 4 volts, indicating that the PCM and CKT 432 are OK.

Diagnostic Aids:
An intermittent open in CKT 432 or CKT 474 will result in a Code 34.

With the ignition "ON" and the engine "OFF," the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the PCM as an indication of vehicle altitude.
Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Reading should be the same + 0.4 volt.
Also, to test the MAP sensor, see Manifold Absolute Pressure Output Check or Intermittents.
See: Symptom Related Diagnostic Procedures\Intermittent Condition

Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than 0.1 volt indicates a bad connector or connection. If OK, replace sensor.

The 54 is:

Circuit Description:
The status of the fuel pump CKT 120 is monitored by the PCM at terminal "F6" and is used to compensate fuel delivery based on system voltage. This signal is also used to store a DTC if the fuel pump relay is defective or fuel pump voltage is lost while the engine is running. There should be about 12 volts on CKT 120 for at least 2 seconds after the ignition is turned "ON," or any time reference pulses are being received by the PCM.

DTC 54 will set if the voltage at "F6" is less than 2 volts for 1.5 seconds since the last reference pulse was received. This DTC is designed to detect a faulty relay, causing extended crank time, and the DTC will help diagnose an engine that "Cranks But Will Not Run."

If a fault is detected during start-up, the "Service Engine Soon" light will stay "ON" until the ignition is cycled "OFF."

Does the fuel pressure drop off at all over time to the point the fuel pump needs to prime the system again to restart the engine?

Here is what the pressure should be and hold:

PROCEDURE

• Turn engine "OFF" and relieve system pressure. Refer to Fuel Pressure Release Procedure.
• Connect the fuel pressure gauge to the pressure connection. Wrap a shop towel around the fitting while connecting the gauge to avoid spillage.
• "START" engine and observe fuel pressure reading. It should be 380-420 kPa (55-61 psi).
• Turn engine "OFF" and relieve fuel pressure.
• Remove fuel pressure gauge.
• Reinstall fuel pressure connection cap.
• "START" engine and check for fuel leaks

Dan39406.0180032407

Expert:	Dan
Pos. Feedback:	99.0 %
Accepts:	2013
Answered:	11/20/2007

Auto Technician

GM Mark of Excellence/ Service Guild Participant with 13 years experience, ASE Master Certified