GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel Lock-Off Relay Diagnosis

Circuit Description

The lock-off relay coil is supplied power whenever the ignition is in the Start or Run position. When the lock-off relay is energized it supplies power to the high pressure lock-off (HPL) solenoid and the low pressure lock-off (LPL) solenoid.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Fuel Injector Control Module Connector End Views

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2
  1. Disconnect the AF lock-off relay.
  2. Probe the relay switch battery circuit terminal (30) with a test lamp connected to ground.

Is the test lamp illuminated?

Go to Step 3

Go to Step 5

3
  1. Probe the relay coil ignition circuit terminal (85) with a test lamp connected to ground.
  2. Turn ON the ignition with the engine OFF.

Is the test lamp illuminated?

Go to Step 4

Go to Step 7

4

Probe the relay coil ground circuit terminal (86) with a test lamp connected to B+.

Is the test lamp illuminated?

Go to Step 12

Go to Step 9

5

Inspect the AFS/4WS fuse.

Is the fuse open?

Go to Step 6

Go to Step 10

6
  1. Test for one of the following conditions:

    2. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

    • A shorted to ground HPL or LPL positive voltage circuit
    • An internally shorted HPL or LPL solenoid
    • A shorted to ground relay switch battery circuit
    • An internally shorted to ground underhood fuse block
  2. Repair the condition or replace the component as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 14

--

7

Inspect the AFS/PCM 1fuse.

Is the fuse open?

Go to Step 8

Go to Step 11

8
  1. Test for one of the following conditions:

    2. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

    • A shorted to ground relay coil positive voltage circuit
    • An internally shorted relay
    • An internally shorted to ground underhood fuse block
  2. Repair the condition or replace the component as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 14

--

9

Repair the relay coil ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 14

--

10

Repair the relay switch power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 14

--

11

Repair the relay coil power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 14

--

12

Test for an intermittent and for a poor connection at the AF lock-off relay. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

Go to Step 14

Go to Step 13

13

Replace the AF lock-off relay.

Did you complete the replacement?

Go to Step 14

--

14
  1. Clear DTC information with a scan tool from the PCM.
  2. Turn the ignition OFF and wait 30 seconds.
  3. Start and idle the engine.
  4. Observe the MIL.
  5. Observe vehicle performance and driveability.

Does the vehicle operate on CNG with normal driveability and no MIL illumination?

System OK

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)