Fuel Lock-Off Relay Diagnosis. GMC Truck Savana (CNG) 2004

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2004 🢒 Savana (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel Lock-Off Relay Diagnosis

Circuit Description

The powertrain control module (PCM) controls the fuel system while the vehicle is operating on CNG. A high pressure lock-off (HPL) solenoid in each CNG tank and a low pressure lock-off (LPL) solenoid in the fuel rail prevent fuel flow. The PCM commands ONLY the HPL's open for 1 second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tanks. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

The lock-off relay's coil and switch are supplied power whenever the ignition is in the Start or Run position. The PCM controls the ground path. When the lock-off relay's are energized they supply 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	Is DTC P0005 or P2665 set?	Go to DTC P0005 or DTC P2665	Go to Step 3
3	Turn ON the ignition with the engine OFF. Command the high pressure lock-off (HPL) ON and OFF with a scan tool. Listen for operation of the HPLs. Do the HPLs respond to the scan tool commands?	Go to Step 4	Go to Step 5
4	Command the low pressure lock-off (LPL) ON and OFF with a scan tool. Listen for operation of the LPL. Does the LPL respond to the scan tool commands?	Go to Step 18	Go to Step 6
5	Command the HPL solenoids ON and OFF with a scan tool. Listen or feel for operation of the HPL relay. Does the HPL relay respond to the scan tool commands?	Go to Step 14	Go to Step 15
6	Command the LPL solenoid ON and OFF with a scan tool. Listen or feel for operation of the LPL relay. Does the LPL relay respond to the scan tool commands?	Go to Step 8	Go to Step 7
7	Disconnect the LPL relay. Probe the relay switch ignition circuit terminal (30) with a test lamp connected to ground. Turn ON the ignition with the engine OFF. Is the test lamp illuminated?	Go to Step 10	Go to Step 11
8	Disconnect the LPL solenoid. Probe the LPL solenoid ground circuit terminal with a test lamp connected to B+. Is the test lamp illuminated?	Go to Step 9	Go to Step 13
9	Probe the LPL solenoid ignition voltage circuit terminal with a test lamp connected to ground. Command the LPL solenoid ON and OFF with a scan tool. Does the test lamp respond to the scan tool commands?	Go to Step 17	Go to Step 12
10	Test for an intermittent and for a poor connection at the LPL relay. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the G/H Van Service Manual. Did you find and correct the condition?	Go to Step 18	Go to Step 16
11	Repair the LPL relay switch power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair?	Go to Step 18	--
12	Repair the LPL solenoid power circuit between the relay and the solenoid for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair?	Go to Step 18	--
13	Repair the LPL solenoid ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair?	Go to Step 18	--
14	Repair the HPL solenoid's ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair?	Go to Step 18	--
15	Replace the HPL relay. Did you complete the replacement?	Go to Step 18	--
16	Replace the LPL relay. Did you complete the replacement?	Go to Step 18	--
17	Replace the LPL solenoid. Refer to Low Pressure Lock-Off Solenoid Replacement . Did you complete the replacement?	Go to Step 18	--
18	Clear DTC information with a scan tool from the PCM. Turn the ignition OFF and wait 30 seconds. Start and idle the engine. Observe the MIL. 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)