Diagnostic Aids

Enter this test only when all steps under QUICK TEST have been successfully completed and engine still does not start or if directed here from another test or chart. This test is only intended to diagnose:

• Solenoid tank valve/lockoff solenoid.
• Alternative fuel lines and filters.
• Compuvalve.

NOTE: When FRPREAB PID values fall below 95 psi (on CNG vehicles) or 25 psi (on LPG vehicles), ALT fuel switch indicator will illuminate.
1. 1) Check Bi-Fuel PID FRPREAB
   Turn ignition switch to OFF position. Connect New Generation Star (NGS) scan tool to Data Link Connector (DLC). Turn ignition switch to ON position (engine off). Using scan tool, access DIAGNOSTIC DATA LINK menu, and select AFCM. Highlight and select PID/DATA MONITOR AND RECORD. Highlight and select FRPREAB PID. If using OBD-II compliant scan tool to view PID values, use scan tool manufacturers instructions to access PIDs. If FRPREAB value is more than 95 psi (on CNG vehicle) or more than 25 psi (on LPG vehicle), replace compuvalve. If FRPREAB value is 95 psi or less (on CNG vehicles), go to next step. If FRPREAB value is 25 psi or less (on LPG vehicles equipped with manual shut off valves or manual tank valve), go to step  3). If FRPREAB value is 25 psi or less (on all other LPG vehicles), go to step  4).
2. 2) Check Bi-Fuel FTP PID For Correct Fuel Pressure Indication
   
   Ensure ignition switch is turned to ON position. Using scan tool, access FTP PID. If FTP value indicates adequate fuel pressure for engine operation, replace CNG regulator. See FUEL PRESSURE REGULATOR under FUEL SYSTEM in REMOVAL, OVERHAUL & INSTALLATION - CNG/LPG (BI-FUEL) article. If FTP value does not indicate adequate fuel pressure for engine operation (on CNG system equipped with manual shutoff valve), go to next step. If FTP value does not indicate adequate fuel pressure for engine operation (on all other CNG vehicles), go to step  4).
3. 3) Check Manual Fuel Shutoff Valve(s)
   
   Check for open manual shutoff valve (quarter turn valve) or manual tank valve(s) (if equipped). If valve(s) are open, go to next step. If valves are not open, open manual shutoff valve (quarter turn valve) or manual tank valve(s) and retest system.
4. 4) Check Solenoid Tank Valve, High Pressure Solenoid Valve Or Lockoff Valve Operation
   
   Turn ignition switch to OFF position. Place ALT fuel switch in ALT position. Turn ignition switch to ON position. Listen for audible click at High Pressure Solenoid (HPS) valve (on F150), solenoid tank valves (if equipped) or lock off valve (on LPG vehicles). If solenoid tank valve(s), HPS valve or lock off valve do not actuate (no audible click is heard), go to next step. If solenoid tank valve(s), HPS valve or lock off valve actuate (audible click), check for correct alternative fuel level or pressure and check for fuel line damage or restrictions. If all checks are okay, replace compuvalve.
CAUTION: Solenoid valves are equipped with diodes. To prevent damage to diode(s), ensure correct polarity before applying voltage. See WIRING DIAGRAMS article.
5. 5) Check Solenoid Tank Valve, High Pressure Solenoid (HPS) Valve Or Lockoff Valve
   Turn ignition switch to OFF position. Disconnect solenoid tank valve connectors and HPS valve connector (on CNG vehicles) or lock off valve solenoid connector (on LPG vehicles). Momentarily connect a fused jumper wire (battery voltage) and a ground wire to each solenoid valve coil. If each solenoid activates when voltage is applied, go to step  20) for Contour. For all other vehicles, go to next step. If solenoids do not activate when voltage is applied, replace inoperative solenoid valve.
6. 6) Check Lock Off Valve Power Circuit For Open
   
   Turn ignition switch to OFF position. Disconnect in-line connector C4302. Disconnect compuvalve in-line connector C4030. On F350 models, disconnect cold start solenoid lock off valve connector C4020. On all models, measure resistance between in-line harness male connector C4302 terminal No. 1 (terminal No. 16 on F350 models) (Pink wire) and compuvalve in-line female harness connector C4030 terminal No. 4 (Pink wire). On F350 models, measure resistance between solenoid lock off valve connector C4020 Terminal No. 1 (Black wire) and compuvalve in-line harness male connector C4003 terminal No. 3 (White wire). Measure resistance between solenoid lock off valve connector C4020 Terminal No. 2 (Black wire) and compuvalve in-line harness male connector C4003 terminal No. 1 (Yellow wire). If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open in affected circuit.
7. 7) Check Compuvalve Main Harness For Opens
   
   Ensure ignition switch is turned to OFF position. Disconnect compuvalve connector C4025. Disconnect compuvalve in-line harness connector C4003. Ensure compuvalve in-line harness connector C4030 is disconnected. Measure resistance between compuvalve in-line harness male connector C4030 terminal No. 4 (White wire) and solenoid lock off valve (on LPG) or HPS valve (on CNG) in-line harness female connector C4003 terminal No. 1 (White wire). Measure resistance between compuvalve in-line harness male connector C4030 terminal No. 3 (White wire) and solenoid lock off valve (on LPG) or HPS valve (on CNG) in-line harness female connector C4003 terminal No. 1 (White wire). Measure resistance between compuvalve in-line harness male connector C4030 terminal No. 3 (White wire) and compuvalve harness connector C4025 terminal No. 3 (White wire). If any measurement is 5 ohms or more, repair open in affected circuit. If all measurements are less than 5 ohms on F150 (LPG), replace compuvalve. If all measurements are less than 5 ohms on F350 models (equipped with dual LPG tanks), reconnect all connectors and go to step  9). If all measurements are less than 5 ohms on CNG vehicles, go to next step.
8. 8) Check Solenoid Tank Valve Circuits For Opens
   
   Ensure ignition switch is turned to OFF position. Disconnect solenoid tank valve harness connector C4120. Ensure compuvalve in-line harness connector C4030 is disconnected. Measure resistance between compuvalve in-line harness female connector C4030 terminal No. 3 (Yellow wire) and lock off valve harness connector C4120. Measure resistance between compuvalve in-line harness female connector C4030 terminal No. 4 (Pink wire) and lock off valve harness connector C4120. If resistances are less than 5 ohms, replace compuvalve. If resistances are 5 ohms or more, repair open in affected circuit.
9. 9) Check Engine Starting With Dual LPG Fuel Tank Switch In Both Positions
   
   Attempt to start engine with dual LPG fuel tank switch in both positions. If engine starts with dual tank switch in either position, go to next step. If engine does not start with dual tank switch in either position, go to step  11).
NOTE: Lock off solenoid connector views are not available. Probe both Black wires to assure correct power or ground wire is tested. DO NOT apply voltage or ground to either circuit or damage may occur to circuits, components or fuses.
10. 10) Check LPG Tank Solenoid Lock Off Valve Circuits For Opens
    Turn ignition switch to OFF position. Disconnect in-line harness connector C4303. Disconnect inoperative solenoid lock off valve connector C4121 (front tank) or C4122 (rear tank). Measure resistance between inoperative lock off solenoid valve return circuit harness connector C4121 or C4122 and in-line harness male connector C4303 terminal No. 3 (Yellow wire). See Figure. Measure resistance between inoperative lock off solenoid harness connector C4121 or C4122 and LPG sender relay No. 2 harness connector C4058 terminal No. 87A (Violet wire). See Figure. If resistances are less than 5 ohms, go to step  14). If resistances are 5 ohms or more, repair open in affected circuit.
11. 11) Check Lock Off Solenoid Valve For Voltage
    
    Ensure LPG sender relay No. 2 harness connector is reconnected. Turn ignition switch to ON position (engine off), while measuring voltage between lock off solenoid valve harness connector C4121 and ground. If voltage is momentarily more than 10 volts when ignition switch is turned to ON position, go to step  13). If voltage does not momentarily go to more than 10 volts when ignition switch is turned to ON position, go to next step.
12. 12) Check LPG Sender Relay No. 2 For Voltage
    
    Disconnect LPG sender relay No. 2 harness connector C4058. Turn ignition switch to ON position (engine off), while measuring voltage between LPG sender relay No. 2 terminal No. 30 (Pink wire) and ground. See Figure. If voltage is momentarily more than 10 volts when ignition switch is turned to ON position, replace LPG sender relay No. 2. If voltage does not momentarily go to more than 10 volts when ignition switch is turned to ON position, repair open in affected circuit.
13. 13) Check Lock Off Solenoid Valve Circuit For Open
    
    Turn ignition switch to OFF position. Disconnect compuvalve in-line harness connector C4030. Measure resistance between compuvalve in-line harness female connector C4030 terminal No. 3 (Yellow wire) and lock off solenoid harness connector C4121 (front tank). If resistance is less than 5 ohms, replace compuvalve. If resistance is 5 ohms or more, repair open in affected circuit.
14. 14) Check LPG Sender Relay No. 2 Circuits For Opens
    
    Ensure ignition switch is turned to OFF position. Disconnect compuvalve in-line harness connector C4036. See Figure. Ensure LPG sender relay No. 2 is disconnected. Measure resistance between LPG sender relay No. 2 harness connector C4058 terminal No. 85 (Black wire) and ground. See Figure. Measure resistance between LPG sender relay No. 2 harness connector C4058 terminal No. 86 (White wire) and in-line harness male connector C4036 terminal No. 5 (White wire). If both measurements are less than 5 ohms, go to next step. If any measurement is 5 ohms or more, repair open in affected circuit.
15. 15) Check LPG Sender Relay
    
    Turn ignition switch to OFF position. Ensure compuvalve in-line harness connectors C4030 and C4036 and lock off solenoid valve harness connector C4122 (rear tank) are disconnected. Reconnect LPG sender relay. Connect a fused jumper wire between in-line harness male connector C4036 terminal No. 5 (White wire) and battery voltage. Measure resistance between compuvalve in-line harness female connector terminal No. 4 (Pink wire) and lock off solenoid harness connector C4122. If resistance is less than 5 ohms, replace compuvalve. If resistance is 5 ohms or more, replace LPG sender relay.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 15) to step 20). No test procedures have been omitted.
16. 20) Check CNG Solenoid Tank Valve Circuit For Opens
    
    Ensure ignition switch is turned to OFF position. Disconnect tank valve harness connector C4120 and in-line harness connectors C4030 and C4420. Measure resistance between in-line harness female connector C4420 terminal No. 3 (Violet/Orange wire) and tank valve harness connector C4120. Measure resistance between tank valve harness connector C4120 and in-line harness male connector C4030 terminal No. 3 (Yellow wire). If measurements are less than 5 ohms, go to next step. If measurements are 5 ohms or more, repair open in affected circuit.
17. 21) Check Compuvalve Harness For Open Circuit
    
    Ensure ignition switch is turned to OFF position. Disconnect compuvalve harness connector C4025. See Figure. Measure resistance between in-line harness female connector C4030 terminal No. 3 (White wire) and compuvalve harness connector C4025 terminal No. 3 (White wire). If resistance is less than 5 ohms, replace compuvalve. If resistance is 5 ohms or more, repair open in affected circuit.