Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Test Procedure
- HF1 DTCS P0420 OR P0430: CHECK FOR MISFIRE DETECTION MONITOR DTCS NOTE: Be sure the customer has not:
- Refueled the vehicle with leaded gasoline.
- Noticed a high vehicle oil consumption. (An engine that consumes oil at a high rate deposits high levels of phosphorus on the catalyst and reduces the catalyst efficiency).
NOTE: Internal deterioration of a catalytic converter is usually caused by abnormal engine operation upstream of the catalyst. Events that can produce higher than normal temperatures in the catalyst are particularly suspect. For example, misfiring can cause higher than normal catalyst operating temperatures.- Check for continuous memory DTCs.
Are DTCs P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0309, P0310, P0315, P0316 or P1309 present?
Yes No GO to POWERTRAIN DTC CHARTS AND DESCRIPTIONS - GASOLINE MODELS for pinpoint test direction and REPAIR the misfire DTCs. GO to HF2. - HF2 CHECK FOR HO2S DTCS NOTE: Incorrect HO2S signal input can cause an abnormal temperature in the catalyst.
- Refer to HEATED OXYGEN SENSOR (HO2S) MONITOR for possible HO2S DTCs.
Are there any HO2S DTCs?
Yes No GO to POWERTRAIN DTC CHARTS AND DESCRIPTIONS - GASOLINE MODELS for pinpoint test direction and REPAIR the HO2S DTCs. GO to HF3. - HF3 CHECK FOR CHT OR ECT SENSOR DTCS NOTE: Temperature sensor DTCs can indicate the thermostat is not operating correctly or the engine coolant level is not filled to specification, producing above normal operation temperatures.
- Check for self-test DTCs.
Are DTCs P0116, P0117, P0118, P0119, P0125, P0128, P0217, P1116, P1117, P1285, P1288, P1289, P1290, P1299 or P128A present?
Yes No GO to POWERTRAIN DTC CHARTS AND DESCRIPTIONS - GASOLINE MODELS for pinpoint test direction and REPAIR the CHT or ECT DTCs. GO to HF4. - HF4 CHECK FOR ANY OTHER DTCS
Are there any other DTCs recorded in HF1 (not including the initial P0420 or P0430)?
Yes No GO to POWERTRAIN DTC CHARTS AND DESCRIPTIONS - GASOLINE MODELS for pinpoint test direction and REPAIR the other DTCs first. GO to HF5. - HF5 CHECK THE ELECTRONIC ENGINE CONTROL (EEC) WIRING HARNESS \PCM NOTE: Check the HO2S electrical connectors to make sure the correct HO2S is connected to the correct electrical connector. The electrical connectors are color coded to make sure the correct connection is made.
If the electrical connection of the rear HO2S are interchanged/crossed, the catalyst efficiency monitor test will fail.
- Visually inspect the vehicle's HO2S harness connectors for any indication of crossed wiring.
- Visually inspect the harness for exposed wiring, corrosion and correct routing.
- Visually inspect the PCM connectors for damaged, or pushed out pins, corrosion and loose wires.
Are there any concerns with the wiring or the PCM connection?
Yes No REPAIR as necessary.
CARRY OUT the catalyst monitor drive cycle to verify the repairs.GO to HF6. - HF6 CHECK THE FUEL PRESSURE WARNING: THE FUEL SYSTEM REMAINS PRESSURIZED WHEN THE ENGINE IS NOT RUNNING. TO PREVENT INJURY OR FIRE, USE CAUTION WHEN WORKING ON THE FUEL SYSTEM.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.