GM Service Manual Online
For 1990-2009 cars only

Object Number: 56429  Size: LF
Engine Controls Components
DIESEL PCM, MAF, EGR, APP, WASTEGATE
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The EGR flow is controlled by the vacuum actuated EGR valve. The EGR pulse width modulated solenoid meters the vacuum level at the EGR valve actuator as commanded by the PCM. When the PCM is commanding the EGR solenoid ON, the voltage potential of the circuit will be low (near 0 volts). When the PCM is commanding the EGR solenoid OFF, the voltage potential of the circuit will be high (near battery volts). The primary function of the PCM in this circuit is to supply the ground for the EGR solenoid. This is a type B code.

Conditions for Setting the DTC

    • PCM requested EGR solenoid ON.
    • Voltage on EGR solenoid control circuit high (near battery volts).
    • 2 consecutive faults detected.
    • Conditions met for 2 seconds.

or

    • PCM requested EGR solenoid OFF.
    • Voltage on EGR solenoid control circuit low (near 0 volts).
    • 2 consecutive faults detected.
    • Conditions met for 2 seconds.

Action Taken When the DTC Sets

The MIL will illuminate on the 2 test fail.

Conditions for Clearing the MIL/DTC

    • The PCM will turn the MIL off after three consecutive trips without a fault condition.
    • A History DTC will clear when forty consecutive warm-up cycles that the diagnostic does not fail (coolant temperature has risen 5°C (40°F) from start up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle.
    • Use of a Scan Tool

Diagnostic Aids

The scan tool has a 5 second ON time abort. The EGR solenoid can be commanded ON for as many times as needed, in 5 second intervals. Its possible other EGR DTCs may set along with DTC P1655. This diagnostic can be checked at key up. A quick operational check can be made by commanding the EGR solenoid ON and OFF with the scan tool while monitoring Actual EGR. Actual EGR will display Baro (approximately) when EGR solenoid is OFF. Full EGR will be achieved when EGR solenoid is commanded ON with the scan tool.

The Adaptive Learn Matrix (ALM) is used to adjust the EGR vacuum control based on Mass Air Flow (MAF). The ALM may change as a result of back pressure increases over the life of the vehicle or other engine system variations. The ALM is made up of sixteen cells (numbered from zero to fifteen) in which each cell covers a range of engine speed (RPM) and load (mm3).

An intermittent may be caused by the following:

    • Poor connections.
    • Rubbed through wire insulation.
    • Broken wire inside the insulation.

Test Description

Number(s) below refer to the Step number(s) on the Diagnostic Table.

  1. Be sure that both the ON and OFF states are commanded. Repeat the commands as mant times as necessary.

  2. This check can detect a partially shorted coil which would cause excessive current flow. Leaving the circuit engerized for 2 minutes allows the coil to warm up. When warm, the coil may open (Amps drop to 'zero', or short (Amp draw greater than 0.75A).

  3. If no trouble is found in the control circuit or the connection at the PCM, the PCM may be malfunctioning, however, this is an extremely unlikely failure.

Step

Action

Value(s)

Yes

No

1

Important: Before clearing DTCs use the scan tool Capture Info to record freeze frame and failure records for reference, as data will be lost when Clear Info function is used.

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Scan tool connected.
  2. Ignition ON, engine OFF.
  3. With scan tool, command EGR solenoid ON and OFFand listen for an audible "click".

Does the EGR solenoid turn ON and OFF (audible "click") with each command?

--

Go to Step 3

Go to Step 5

3

  1. Ignition OFF.
  2. Disconnect the PCM connector containing the EGR solenoid control circuit.
  3. Ignition ON.
  4. Using DVM J 39200 on 10 Amp scale, measure current from the solenoid control circuit in the PCM harness connector to ground for 2 minutes.

Is current draw less then the specified value, but not zero?

0.75 A

Go to Step 8

Go to Step 4

4

  1. Ignition OFF.
  2. PCM connector still disconnected.
  3. Disconnect EGR solenoid.
  4. Using DVM J 39200 , measure resistance from the solenoid control circuit in the PCM harness connector to ground.

Does DVM display infinite resistance?

--

Go to Step 13

Go to Step 10

5

  1. Disconnect EGR solenoid
  2. Ignition ON, engine OFF.
  3. Connect a test light between the terminals at the harness connector.
  4. Using a scan tool, command the solenoid ON and OFF.

Does test light turn ON and OFF with each command?

--

Go to Step 9

Go to Step 6

6

  1. Ignition ON engine OFF.
  2. With a test light connected to ground, probe the ignition feed circuit at the EGR solenoid harness connector.

Is the test light ON?

--

Go to Step 7

Go to Step 12

7

  1. Ignition OFF.
  2. Reconnect solenoid.
  3. Disconnect the PCM harness containing the solenoid control circuit.
  4. Ignition ON.
  5. With a fused jumper wire connected to ground, probe the solenoid control circuit in the PCM harness connector.

Does the solenoid operate (audible "click")?

--

Go to Step 11

Go to Step 10

8

DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs were stored refer to the applicable DTC table(s) first.

Are any additional DTCs stored?

--

Go to the Applicable DTC Table

Go to Diagnostic Aids

9

Check for a poor connection at the EGR solenoid and replace terminals as necessary.

Did the terminals require replacement?

--

Go to Step 15

Go to Step 13

10

Repair EGR solenoid control circuit.

Is the action complete?

--

Go to Step 15

--

11

Check for a poor connection at the PCM, EGR solenoid control circuit.

Was a problem found?

--

Go to Step 15

Go to Step 14

12

Repair the open in the ignition feed circuit.

Is the action complete?

--

Go to Step 15

--

13

Replace the EGR solenoid.

Is the action complete?

--

Go to Step 15

--

14

Replace the PCM.

Important: The new PCM must be programmed.

Refer to Powertrain Control Module Replacement/Programming .

Is the action complete ?

--

Go to Step 15

--

15

  1. Using the scan tool, select DTC, Clear Info.
  2. Start engine and idle at normal operating temperature.
  3. Select DTC, Specific, then enter the DTC number which was set.
  4. Operate vehicle untill the scan tool indicates that the diagnostic Ran.

Does the scan tool indicate that this diagnostic Passed?

--

Go to Step 15

Go to Step 2

16

Using the scan tool, select Capture Info, Review Info.

Are any DTCs displayed that have not been diagnosed?

--

Go to the Applicable DTC Table

System OK