Diagnostic Aids

Check for a faulty light bulb or open in Red wire circuit. If engine cranks, but will not start, check battery feed circuit, EFI main relay, ignition circuit, and ECM connections.

Fig 1: Flow Chart A1, No Check Engine Light

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Fig 2: Chart A1 Schematic, No Check Engine Light

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

Fig 1: Code 12 & 13 Flow Chart, No RPM Signal/No Signal For 2 Seconds

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Fig 2: Code 12 & 13 Schematic, No RPM Signal/No Signal For 2 Seconds

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An intermittent condition may be caused by rubbeDthrough insulation or a wire broken inside the insulation. Check ECM harness connectors for backeDout or damaged terminals. Ensure tachometer is operational in instrument panel. If tachometer is not operational, check ignitor terminal IGT of ECM for an open or short circuit. If code is intermittent, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 14 Flow Chart, Ign. Signal Circuit/No Signal

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Fig 2: Code 14 Schematic, Ign. Signal Circuit/No Signal

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Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 will set in one minute if sensor signal voltage remains between 0-1 volt. Verify clean tight connection on Black wire. An open Red or Black wire will result in Code 21. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 21 Flow Chart, O2 Sensor Circuit/Open or Short Ckt.

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Fig 2: Code 21 Schematic, O2 Sensor Circuit/Open or Short Ckt.

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After engine is started, engine coolant temperature should rise to approximately 203°F (95°C), and stabilize when thermostat opens. Code 22 will set if Brown or White wire is open. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 22 Flow Chart, Coolant Temp. Sensor/Open or Short Ckt.

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Fig 2: Code 22 Schematic, Coolant Temp. Sensor/Open or Short Ckt.

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An intermittent condition may be caused by rubbeDthrough insulation, or a wire broken inside the insulation. Check ECM harness connectors for backeDout or damaged terminals. Check for damaged Mass Airflow (MAF) Sensor housing. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 24 Flow Chart, Intake Air Temp. Sensor Ckt./Low Temp. Indicated

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Fig 2: Code 24 Schematic, Intake Air Temp. Sensor Ckt./Low Temp. Indicated

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Check O2 sensor wire for contact with exhaust manifold. Check O2 wire for ground between connector and sensor. Check for fuel contamination. A small amount of water in fuel tank can be delivered to the fuel injector, causing Code 25 to set. Check fuel pressure. See BASIC TESTING article in the ENGINE PERFORMANCE Section. Check for exhaust leaks. An exhaust leak can cause outside air to be pulled into the exhaust past the O2 sensor. Vacuum or crankcase leaks can cause a lean condition. If Code 25 is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 25 Flow Chart, O2 Sensor Ckt./Lean Exhaust Indicated

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Fig 2: Code 25 Schematic, O2 Sensor Ckt./Lean Exhaust Indicated

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If fuel system pressure goes high, ECM can compensate for some increase; however, if fuel pressure goes too high, Code 26 will set. See BASIC TESTING article in the ENGINE PERFORMANCE Section. A leaking fuel injector can cause this code to set. See BASIC TESTING article in the ENGINE PERFORMANCE Section.

A faulty canister purge system can cause Code 26. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section. A faulty MAF sensor can cause this code to set. Disconnect MAF sensor connector. This allows the ECM to substitute a fixed MAF sensor signal value. If rich condition is no longer present, faulty MAF sensor is the cause of rich condition.

An intermittent throttle switch output will cause the system to operate rich due to a false signal of engine acceleration. Inspect O2 sensor for silicone contamination from fuel, or improper use of RTV sealant. The sensor may have WhitEpowdery coating, which can result in a high, but false, signal voltage (rich condition). Check for a sticking EGR valve at idle. If Code 26 is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 26 Flow Chart, O2 Sensor Ckt./Rich Exhaust Indicated

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Fig 2: Code 26 Schematic, O2 Sensor Ckt./Rich Exhaust Indicated

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Ensure suBO2 sensor has a clean, tight connection to Black wire. Open in the Black or White wire will result in Code 27. If Code 27 is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 27 Flow Chart, Sub-O2 Sensor/Open or Shorted Ckt. (Calif. Only)

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Fig 2: Code 27 Schematic, Sub-O2 Sensor/Open or Shorted Ckt. (Calif. Only)

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Intermittent condition may be caused by rubbeDthrough wire insulation, broken wire inside the insulation, or poor connection. Inspect ECM harness connectors for backeDout terminals. Check for damaged MAF housing.

Fig 1: Code 31 Flow Chart, MAF Sensor/Short Circuit

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Fig 2: Code 31 Schematic, MAF Sensor/Short Circuit

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Intermittent condition may be caused by rubbeDthrough wire insulation, wire broken inside insulation, or poor connection. Inspect ECM harness connector for backeDout terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness. If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 41 Flow Chart, TPS Sensor/Open or Short Circuit