Diagnostic Aids

If CHECK ENGINE light will not illuminate, check for faulty bulb or an open circuit in Violet wire. If engine cranks but will not start, the following are possible causes: battery feed circuit open, control relay faulty, ignition circuit open, or poor ECM connections.

Fig 1: Chart A-1, Flow Chart, No Check Engine Light

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Fig 2: Chart A-1, Schematic, No Check Engine Light

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Normal O2 sensor voltage will vary from 0-.9 volt. Code 13 will be set if voltage is low or high all the time. A loose or intermittent O2 sensor ground can cause a Code 13. O2 sensor ground is located on lower right rear of intake manifold. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 13, Flow Chart, O2 Sensor Ckt. Signal Voltage Steady

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Fig 2: Code 13, Schematic, O2 Sensor Ckt. Signal Voltage Steady

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A faulty connection or open in Gray/White or Lt. Green/Black wire could result in Code 14. If Codes 14 and 22 are stored at the same time, problem may be an open Lt. Green/Black wire. A shifted sensor may cause poor driveability. Use temperature and resistance values of CTS from DIAGNOSTIC AID table, in CODE 15 chart, to check for a shifted sensor. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 14, Flow Chart, Coolant Temp. Sensor Ckt. Low Temp. Indicated

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Fig 2: Code 14, Schematic, Coolant Temp. Sensor Ckt. Low Temp. Indicated

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Check Gray/White wire for a short to ground. Ensure coolant system is functioning properly. Ensure engine is not overheating, or subjected to conditions such as trailer towing, which will cause engine to run too hot. A shifted sensor may cause poor driveability. Use temperature and resistance values of CTS from DIAGNOSTIC AID table to check for a shifted sensor. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 15, Flow Chart, Coolant Temp. Sensor Ckt. High Temp. Indicated

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Fig 2: Code 15, Schematic, Coolant Temp. Sensor Ckt. High Temp. Indicated

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An intermittent may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 21, Flow Chart, Throttle Switch Ckt. (M/T) Idle Switch & WOT Switch Indicated On

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Fig 2: Code 21, Schematic, Throttle Switch Ckt. (M/T) Idle Switch and WOT Switch Indicated On

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See ADJUSTMENTS article in the ENGINE PERFORMANCE Section for TPS adjustment. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 21, Flow Chart, TPS Circuit (A/T), Signal Voltage High

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Fig 2: Code 21, Schematic, TPS Circuit (A/T), Signal Voltage High

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An intermittent may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 22, Flow Chart, TPS Circuit (A/T) Signal Voltage Low

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Fig 2: Code 22, Schematic, TPS Circuit (A/T) Signal Voltage Low

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An intermittent may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 23, Flow Chart, Intake Air Temp Sensor Ckt. Low Temp Indicated

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Fig 2: Code 23, Schematic, Intake Air Temp Sensor Ckt. Low Temp Indicated

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An intermittent may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If no trouble is found but Code 24 resets, replace ECM.

Fig 1: Code 24, Flow Chart, Vehicle Speed Sensor Circuit

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Fig 2: Code 24, Schematic, Vehicle Speed Sensor Circuit

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An intermittent condition may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 25, Flow Chart, Intake Air Temp Sensor Ckt. High Temp Indicated

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Fig 2: Code 25, Schematic, Flow Chart, Intake Air Temp Sensor Ckt. High Temp Indicated

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An intermittent condition may be caused by rubbed-through insulation, or a broken wire inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 31, Flow Chart, MAP Sensor Ckt. Signal Voltage Low W/High Vacuum

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Fig 2: Code 31, Schematic, MAP Sensor Ckt. Signal Voltage Low W/High Vacuum

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An intermittent condition may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 32, Flow Chart, MAP Sensor Ckt. Signal Voltage High W/Low Vacuum

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Fig 2: Code 32, Schematic, MAP Sensor Ckt. Signal Voltage High W/Low Vacuum

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An intermittent condition may be caused by rubbed-through insulation, or a wire broken inside wire insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 41, Flow Chart, Ign Signal Ckt., LSi Std. Ign. W/Vacuum Advance

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Fig 2: Code 41, Schematic, Ign Signal Ckt., LSi Std. Ign. W/Vacuum Advance

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An intermittent condition may be caused by rubbed-through insulation or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 41, Flow Chart, Ign. Signal Ckt. No Signal, XFi W/ESC

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Fig 2: Code 41, Schematic, Ign. Signal Ckt. No Signal, XFi W/ESC

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An intermittent condition may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 42, Flow Chart, Crank Angle Sensor Ckt., No Signal For 2 Seconds, XFi W/ESA Ignition

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Fig 2: Code 42, Schematic, Crank Angle Sensor Ckt., No Signal For 2 Seconds, XFi W/ESA Ignition

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An intermittent condition may be caused by rubbed-through insulation, or a wire broken inside the insulation. Check ECM harness connectors for backed-out or damaged terminals. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 51, Flow Chart, EGR Ckt. (Calif. & XFi Only)

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Fig 2: Code 51, Schematic, EGR Ckt. (Calif. & XFi Only)

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