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Diagnostic Aids

PCM determines engine cooling fan requirements and controls fan operation through Low Fan Control (LFC) and High Fan Control (HFC) circuits. HFC relay is normally open. LFC relay is normally open.

Perform this test only when instructed by QUICK TEST  or directed by other test procedures. This test is only intended to diagnose:

Fig 1: LFC/HFC Circuits
G95I60127
NOTE: During diagnosis, use FAN CONTROL CIRCUIT TEST PIN IDENTIFICATION  table to determine correct breakout box test pin, circuit and relay being tested.
FAN CONTROL CIRCUIT TEST PIN IDENTIFICATION

Application Circuit Breakout Box Test Pin No.
Code P1474 LFC 45
Code P1479 HFC 17
  1. Step 1) Code P1474 Or P1479  Code P1474 indicates that circuit failure has occurred on Low Fan Control (LFC) circuit (Fan Control circuit for single speed fan applications). Code P1479 indicates that circuit failure has occurred on High Fan Control (HFC) circuit. Possible causes for this fault are:
    • Open or shorted FC, LFC or HFC circuit.
    • Open IGN RUN/START circuit to fan control relay.
    • Faulty fan control relay.
    • Faulty PCM.

    Turn ignition off. Disconnect appropriate fan control relay. Turn ignition on. Measure voltage between negative battery terminal and IGN RUN/START circuit terminal at cooling fan relay wiring harness connector. See Fig 1. If voltage is more than 10.5 volts, go to next step. If voltage 10.5 volts or less, locate and repair open IGN START/RUN circuit. Reconnect cooling fan relay and repeat QUICK TEST .

  2. Step 2) Check FC, LFC Or HFC Circuit  Turn ignition off. Ensure relay being tested is disconnected. Connect NGS scan tester to Data Link Connector (DLC). Turn ignition on. Access Output Test Mode on NGS scan tester. Connect DVOM positive lead to IGN START/RUN circuit terminal of cooling fan relay wiring harness connector. Connect DVOM negative lead to FC, LFC or HFC circuit terminal of appropriate fan control relay wiring harness connector. Observe DVOM and turn cooling fan on and off several times. If voltage does not change more than 0.5 volt when cooling fan is turned from off to on, go to next step. If voltage changes more than 0.5 volt when cooling fan is turned from off to on, replace appropriate fan control relay. Repeat QUICK TEST .
  3. Step 3) Check For Short To Power In FC, LFC Or HFC Circuit  Turn ignition off. Disconnect appropriate fan control relay. Disconnect PCM connector. Inspect connector terminals. Clean or repair connector as necessary. Turn ignition on. Measure voltage between negative battery terminal and FC, LFC or HFC circuit terminal at cooling fan relay wiring harness connector. If voltage is less than one volt, go to next step. If voltage is one volt or more, locate and repair short to power in FC, LFC or HFC circuit. Reconnect all components and repeat QUICK TEST .
  4. Step 4) Check For Short To Ground In FC, LFC Or HFC Circuit  Turn ignition off. Disconnect NGS scan tester from Data Link Connector (DLC). Ensure appropriate fan control relay and PCM are disconnected. Install breakout box, leaving PCM disconnected. Measure resistance between appropriate fan control circuit (FC, LFC or HFC) test pin and test pins No. 51, 103 (PWR GND), and 91 (SIG RTN) at breakout box. See FAN CONTROL CIRCUIT TEST PIN IDENTIFICATION  table. If all readings are more than 10,000 ohms, go to next step. If any reading is 10,000 ohms or less, locate and repair short to ground in appropriate circuit. Reconnect all components and repeat QUICK TEST .
  5. Step 5) Check FC, LFC Or HFC Circuit Continuity  Measure resistance between appropriate fan control circuit (FC, LFC or HFC) test pin at breakout box and FC, LFC or HFC circuit terminal at fan control relay wiring harness connector. If resistance is less than 5 ohms, replace PCM. Remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is 5 ohms or more, locate and repair open FC, LFC or HFC circuit. Remove breakout box, reconnect all components and repeat QUICK TEST .
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 5) to step 10). No test procedures have been omitted.
  6. Step 10) Continuous Memory Code P1474  Continuous Memory Code P1474 indicates that a failure has occurred in circuit LFC. Possible causes for this fault are:
    • Open IGN START/RUN circuit to LFC relay.
    • Open or short in LFC circuit.

    Turn ignition off. Ensure A/C and defroster are off. Connect NGS scan tester to Data Link Connector (DLC). Disconnect cooling fan motor connector(s). Inspect connector terminals. Clean or repair connector(s) as necessary. Connect a non-powered test light between Power-To-Low Fan circuit and ground circuit at fan motor wiring harness connector. Turn ignition on. Using scan tester, access Output Test Mode. Command Low Speed Fan On. Test light should be on. Observe test light for indication of fault (test light will turn off to indicate fault) while performing the following:

    • Shake and bend FC or LFC circuit between PCM and fan control relay.
    • Shake and bend VPWR or IGN START/RUN circuit between EEC-V power relay and fan control relay.
    • Lightly tap on fan control relay to simulate road shock.

    If fault is indicated, isolate and repair open or short to power in wiring harness. Repeat QUICK TEST . If fault is not indicated, go to next step.

  7. Step 11) Check FC Or LFC Circuit For Short To Ground  Using scan tester, command Low Speed Fan Off. Test light should be off. Observe test light for indication of fault (test light will turn on to indicate fault) while performing the following:
    • Shake and bend FC or LFC circuit between PCM and fan control relay.
    • Lightly tap on fan control relay to simulate road shock.

    If fault is indicated, isolate and repair short to ground in wiring harness. Repeat QUICK TEST . If fault is not indicated, problem is intermittent and cannot be duplicated at this time.

    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 11) to step 20). No test procedures have been omitted.
  8. Step 20) Continuous Memory Code P1479  Continuous Memory Code P1479 indicates that a failure has occurred in High Fan Control (HFC) circuit.

    Possible causes for this fault are:

    • Open IGN START/RUN circuit to HFC fan control relay.
    • Open or short in HFC circuit.

    Turn ignition off. Ensure A/C and defroster are off. Disconnect cooling fan motor connector(s). Inspect connector terminals. Clean or repair connector(s) as necessary. Connect a non-powered test light between Power-To-High Fan circuit and ground circuit at fan motor wiring harness connector. Turn ignition on. Using scan tester, access Output Test Mode. Command High Speed Fan On. Test light should be on. Observe test light for indication of fault (test light will turn off to indicate fault) while performing the following:

    • Shake and bend HFC circuit between PCM and HFC fan control relay.
    • Shake and bend VPWR or IGN START/RUN circuit between EEC-V power relay and HFC fan control relay.
    • Lightly tap on HFC fan control relay to simulate road shock.

    If fault is indicated, isolate and repair open or short to power in wiring harness. Repeat QUICK TEST . If fault is not indicated, go to next step.

  9. Step 21) Check HFC Circuit For Short To Ground  Using scan tester, command High Speed Fan Off. Test light should be off. Observe test light for indication of fault (test light will turn on to indicate fault) while performing the following:
    • Shake and bend HFC circuit between PCM and fan control relay.
    • Lightly tap on HFC fan control relay to simulate road shock.

    If fault is indicated, isolate and repair short to ground in wiring harness. Repeat QUICK TEST . If fault is not indicated, problem is intermittent and cannot be duplicated at this time.

    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 21) to step 25). No test procedures have been omitted.
  10. Step 25) Cooling Fan Inoperative  Perform any cooling fan related codes prior to performing this step. Ignition should still be on, NGS scan tester still connected to Data Link Connector (DLC) and scan tester in Output Test Mode. Disconnect cooling fan motor connector. Using scan tester, command Cooling Fan On. Measure voltage between Power-To-Fan circuit terminal and ground circuit terminal at cooling fan wiring harness connector. See Fig 1. If voltage is more than 10.5 volts, replace cooling fan motor and recheck cooling fan operation. If voltage is 10.5 volts or less, remain in Output Test Mode and go to next step.
  11. Step 26) Check Cooling Fan Motor Ground Circuit  Measure voltage between negative battery terminal and Power-To-Fan circuit terminal at cooling fan motor wiring harness connector. See Fig 1. If voltage is less than 10.5 volts, go to next step. If voltage is 10.5 volts or more, locate and repair open in cooling fan motor ground circuit. Recheck cooling fan operation.
  12. Step 27) Check B + Circuit To Fan Control Relay  Turn ignition off. Measure voltage between negative battery terminal and B + circuit terminal at fan control relay wiring harness connector. See Fig 1. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, turn ignition on and retest. If voltage is still 10.5 volts or less, locate and repair open B + circuit to fan control relay. Recheck cooling fan operation.
  13. Step 28) Check Power-To-Fan Circuit  Turn ignition off. Disconnect fan control relay. Measure resistance of Power-To-Fan circuit between fan control relay wiring harness connector and fan control relay connector. If resistance is less than 5 ohms, replace fan control relay and recheck cooling fan operation. If resistance is 5 ohms or more, locate and repair open in Power-To-Fan circuit. Recheck cooling fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 28) to step 30). No test procedures have been omitted.
  14. Step 30) Check Cooling Fan Operation  During operational check, if cooling fan operated at one speed only, go to step 40). If cooling fan did not operate at any speed, go to next step.
  15. Step 31) Check Voltage To Cooling Fan  Ensure ignition is still on, NGS scan tester is still connected to Data Link Connector (DLC) and in the Output Test Mode, and fan commanded off. Disconnect cooling fan motor connector. Using scan tester, command High Speed Cooling Fan On. Measure voltage between ground and Power-To-Fan circuit terminal at cooling fan wiring harness connector. If voltage is 10 volts or less, go to next step. If voltage is more than 10 volts, go to step 35).
  16. Step 32) Check B + Circuit  Turn ignition off. Disconnect High Fan Control (HFC) relay. Measure voltage between ground and B + circuit at HFC relay. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair B + or To Ignition Relay circuit.
  17. Step 33) Check Power-To-Fan Circuits  Turn ignition off. Disconnect HFC and LFC relays. Measure resistance of Power-To-High Fan circuit between HFC relay wiring harness connector and cooling fan wiring harness connector. Also, measure resistance of Power-To-Low Fan circuit between LFC relay wiring harness connector and cooling fan wiring harness connector. If both readings are less than 5 ohms, replace HFC relay and recheck cooling fan operation. If one or both readings are 5 ohms or less, locate and repair open in Power-To-High Fan circuit or Power-To-Low Fan circuit. Check cooling fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 33) to step 35). No test procedures have been omitted.
  18. Step 35) Check Cooling Fan Ground Circuit  Turn ignition off. Disconnect cooling fan. Disconnect NGS scan tester. Measure resistance between ground and ground circuit terminal at cooling fan wiring harness connector. If resistance is less than 5 ohms, replace cooling fan motor and recheck cooling fan operation. If resistance is 5 ohms or more, locate and repair open ground circuit. Recheck cooling fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 35) to step 40). No test procedures have been omitted.
  19. Step 40) Determine Which Fan Speed Is Operational  If low speed fan was inoperative in step 30), go to next step. If high speed fan was inoperative in step 30), go to step 46).
  20. Step 41) Low Fan Speed Inoperative  Ensure ignition is still on, NGS scan tester is still connected to Data Link Connector (DLC) and in Output Test Mode, and fan commanded off. Disconnect cooling fan motor connector. Using scan tester, command Low Speed Fan On. Measure voltage between ground and Power-To-Low Fan circuit terminal cooling fan wiring harness connector. If voltage is 10 volts or less, go to next step. If voltage is more than 10 volts, verify ground circuit to cooling fan motor. If ground is okay, replace fan motor and recheck cooling fan operation.
  21. Step 42) Check Voltage From HFC Relay-To-LFC Relay  Disconnect LFC relay. Turn ignition on. Measure voltage between ground and HFC Relay circuit terminal at LFC relay wiring harness connector. If voltage is less than 10 volts, go to next step. If voltage is 10 volts or more, go to step 44).
    Fig 2: HFC Relay Circuit
    G95J60128
  22. Step 43) Check Continuity Between LFC & HFC Relays  Turn ignition off. Disconnect LFC and HFC relays. Measure resistance of HFC Relay circuit between LFC relay wiring harness connector and HFC relay wiring harness connector. If resistance is less than 5 ohms, replace HFC relay and recheck fan operation. If resistance is 5 ohms or more, locate and repair open in HFC Relay circuit between LFC and HFC relays.
  23. Step 44) Check Power-To-Low Fan Circuit  Turn ignition off. Ensure LFC relay is disconnected. Disconnect cooling fan connector. Measure resistance of Power-To-Low Fan circuit between LFC relay wiring harness connector and cooling fan connector. If resistance is less than 5 ohms, replace LFC relay and recheck fan operation. If resistance is 5 ohms or more, locate and repair open in Power-To-Low Fan circuit and recheck fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 44) to step 46). No test procedures have been omitted.
  24. Step 46) High Fan Speed Inoperative  Ensure ignition is still on, NGS scan tester is still connected to Data Link Connector (DLC) and in Output Test Mode, and fan commanded off. Disconnect cooling fan motor connector. Using scan tester, command High Speed Fan On. Measure voltage between ground and Power-To-High Fan circuit terminal cooling fan wiring harness connector. If voltage is 10 volts or less, go to next step. If voltage is more than 10 volts, verify ground circuit to cooling fan motor. If ground is okay, replace fan motor and recheck cooling fan operation.
  25. Step 47) Check Power-To-High Fan Circuit  Turn ignition off. Disconnect HFC relay. Measure resistance of Power-To-High Fan circuit between HFC relay wiring harness connector and cooling fan wiring harness connector. If resistance is less than 5 ohms, replace HFC relay and recheck fan operation. If resistance is 5 ohms or more, locate and repair open in Power-To-High Fan circuit. Recheck fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 47) to step 51). No test procedures have been omitted.
  26. Step 51) Check Power-To-Fan Circuit  Measure resistance between ground and Power-To-Fan circuit terminal and both LFC and HFC relays. If both readings are less than 15 ohms, go to next step. If one or both readings are 15 ohms or more, go to step 56).
  27. Step 52) Check For Short To Ground  Disconnect cooling fan motor(s). Measure resistance between ground and Power-To-Fan circuit terminal at HFC relay. If resistance is more than 10,000 ohms, go to next step. If resistance is 10,000 ohms or less, locate and repair short to ground in Power-To-Fan circuit. Recheck fan operation.
  28. Step 53) Check HFC & LFC Relay Operation  Reconnect HFC and LFC relays. With NGS scan tester connected to Data Link Connector (DLC) and in Output Test Mode, check relays as follows:
    • HFC Relay Using scan tester, command High Speed Fan On. Wait 10 seconds. Measure voltage between ground and Power-To-Fan circuit terminal at cooling fan wiring harness connector. Voltage should be more than 10.5 volts.
    • LFC Relay Using scan tester, command Low Speed Fan On. Measure voltage between ground and Power-To-Fan circuit terminal at cooling fan wiring harness connector. Voltage should be more than Step 10.5 volts.

    If both voltage readings are 10.5 volts, replace cooling fan motor(s). Recheck fan operation. If one or both readings are 10.5 volts or less, replace LFC and/or HFC relay. Recheck fan operation.

    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 53) to step 56). No test procedures have been omitted.
  29. Step 56) Check Power-To-Fan Circuit Continuity  Turn ignition off. Disconnect HFC and LFC relays. Disconnect cooling fan motor(s). Measure resistance of Power-To-Fan circuit between cooling fan motor wiring harness connector and both HFC and LFC relay wiring harness connectors. If both readings are less than 15 ohms, go to next step. If one or both readings are 15 ohms or more, locate and repair open in Power-To-Fan circuit.
  30. Step 57) Check Cooling Fan Ground Circuit  Measure resistance between ground and ground circuit terminal at cooling fan wiring harness connector. If resistance is less than 5 ohms, replace inoperative cooling fan motor(s). Recheck fan operation. If resistance is 5 ohms or more, locate and repair open in ground circuit.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 57) to step 60). No test procedures have been omitted.
  31. Step 60) Cooling Fan Always On  Service any Key On Engine Off (KOEO) codes before proceeding with this test step. See KOEO SELF-TEST . If no codes are present, disconnect Fan Control (FC) relay. Turn ignition on. Check cooling fan operation. If cooling fan is still on, locate and repair short to power on Power-To-Fan circuit. Recheck fan operation. If cooling fan is off, replace FC relay and recheck fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 60) to step 65). No test procedures have been omitted.
  32. Step 65) Low &/Or High Speed Cooling Fan Always On  Ensure A/C Pressure Switch (ACPSW) is okay before proceeding with this test step. Turn A/C and defroster off. Go to next step.
  33. Step 66) Check LFC Relay  Turn ignition off. Disconnect LFC relay. Turn ignition on. Check cooling fan operation. If cooling fan is still on, go to next step. If cooling fan is off, replace LFC relay and recheck system operation.
  34. Step 67) Check HFC Relay  Turn ignition off. Disconnect HFC relay. Turn ignition on. Check cooling fan operation. If cooling fan is still on, go to next step. If cooling fan is off, replace HFC relay and recheck system operation.
  35. Step 68) Check For Short To Battery  Disconnect cooling fan motor connector. Turn ignition on. Measure voltage between ground and Power-To-Low Fan circuit terminal at cooling fan wiring harness connector. If voltage is less than one volt, locate and repair short to battery in Power-To-High Fan circuit. If voltage is one volt or more, locate and repair short to battery in Power-To-Low Fan circuit. Recheck fan operation.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 68) to step 70). No test procedures have been omitted.
  36. Step 70) One Cooling Fan Operative & Other Cooling Fan Inoperative  Turn ignition off. Disconnect both cooling fans. Connect NGS Scan Tester to Data Link Connector (DLC). Access Output Test Mode. Command High Speed Fan On. Measure voltage between Power-To-Fan circuit and ground circuit at inoperative cooling fan wiring harness connector. If voltage is 10 volts or less, stay in Output Test Mode, and go to next step. If voltage is more than 10 volts, replace cooling fan motor and recheck fan operation.
  37. Step 71) Check Power-To-Fan Circuit  Measure voltage between Power-To-Fan circuit and chassis ground. If voltage is 10 volts or less, locate and repair open in Power-To-Fan circuit and recheck fan operation. If voltage is more than 10 volts, locate and repair open in cooling fan ground circuit and recheck fan operation.
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