Diagnostic Aids

Check for a faulty light bulb or open in Red wire circuit. Check if 5-volt reference signal for MAP sensor is shorted to ground. If engine cranks, but will not run, check battery feed circuit, EFI main relay, ignition circuit and poor 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, refer to 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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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, refer to TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 14 Flow Chart, Ign. Signal Ckt., Open Or Short Ckt.

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Fig 2: Code 14 Schematic, Ign. Signal Ckt., Open Or Short Ckt.

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Normal O2 sensor voltage varies between .1-1.0 volt. Code 21 will be set in one minute if sensor signal voltage remains between 0-1 volt. Verify clean, tight connection on Brown wire. An open Black or Brown 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 Ckt./Open or Shorted Ckt.

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Fig 2: Code 21 Schematic, O2 Sensor Ckt./Open or Shorted 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 Ckt./Open Or Shorted Ckt.

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Fig 2: Code 22 Schematic, Coolant Temp. Sensor Ckt./Open Or Shorted Ckt.

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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 24 Flow Chart, IAT Sensor Ckt./Low Temp. Indicated

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Fig 2: Code 24 Schematic, IAT 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 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 becomes high, the ECM can compensate for some increase; however, if fuel pressure becomes 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 to set. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section. A faulty MAP sensor can cause this code to set. Disconnect MAP sensor connector. This allows the ECM to substitute a fixed MAP sensor signal value. If rich condition is gone, a faulty MAP 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 cause 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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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 31 Flow Chart, MAP Sensor/Open Or Short Ckt.

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Fig 2: Code 31 Schematic, MAP Sensor/Open Or Short Ckt.3

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Intermittent condition may be caused by rubbed through 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 41 Flow Chart, Throttle Switch Circuit

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Fig 2: Code 41 Schematic, Throttle Switch Circuit

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Intermittent 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 42 Flow Chart, Vehicle Speed Sensor Cicuit

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Fig 2: Code 42 Schematic, Vehicle Speed Sensor Cicuit

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If code is intermittent, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Fig 1: Code 51 Flow Chart, No A/C Signal

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Fig 2: Code 52 Schematic, No A/C Signal

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