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Home >> Mazda >> 2003 >> MX-5 Miata Base, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0420: Catalyst System Efficiency Below Threshold >> Diagnostic Procedure
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Diagnostic Procedure

  1. Verify Related Repair Information Availability -  Turn ignition off. Connect scan tool to DLC-2. See Figure. Turn ignition on, engine off. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check service bulletins for repair information related to this DTC. If repair information is not available, go to next step. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  7.
  2. Verify Related Pending Code Or Stored DTCs -  Stop engine and leave ignition on. Monitor for DTC or pending coderelated to this DTC. If no other DTC is present, go to next step. If other DTC related to this DTCis present, follow appropriate DTC diagnostic procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  3. Inspect Exhaust System For Gas Leakage -  Visually inspect exhaust system for gas leakage. If no exhaust gas leaks are found, go to next step. If any exhaust gas leaks are found, repair or replace faulty exhaust system components as necessary. After repair, go to step  7.
  4. Inspect Installation Of HO2S (Front) & HO2S (Rear) -  Turn ignition off. Inspect front and rear HO2Ss for looseness. If no problem is found, go to next step. If problem is found, resecure suspect front or rear HO2S and/or repair exhaust leak as necessary. After repair, go to step  7.
  5. Inspect HO2S (Front) Voltage -  Start engine and let idle until normal operating temperature is reached. Using scan tool, access O2S11 PID. Verify O2S11 PID while racing engine (in park). Ensure that each PID reading is within following conditions:
    • More than 0.45 volt when suddenly depressing accelerator pedal (rich condition).
    • Less than 0.45 volt just after release of accelerator pedal (lean condition).

    If O2S11 PID readings are as specified, go to next step. If any O2S11 PID reading is not as specified, replace suspect H2OS. After repair, go to step  7.

  6. Check TWC -  Turn ignition off. Allow exhaust system to cool down. Remove TWC and perform inspection. See WARM UP THREE-WAY CATALYTIC CONVERTER under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - MAZDA6, MIATA, MPV, PROTEGE & PROTEGE5 article. If problem is found, replace TWC. After repair, go to next step. If no problem is found, replace rear HO2S. After repair, go to next step.
  7. Verify Troubleshooting Of DTC P0420 Completed -  Return vehicle to original condition. Turn ignition on, engine off. Using scan tool, clear DTCs from PCM memory. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine and perform DRIVE MODES 1, 2 and 3. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM.Monitor for pending DTCs. If same DTC is not present, go to next step. If same DTC P0420 is present, replace PCM. For PCM removal and installation, see POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION article. After repair, go to next step.
  8. Verify After Repair Procedure -  Using scan tool, perform After Repair Procedure. See AFTER REPAIR PROCEDURE . If no other DTCs are present, testing is complete. If any other DTC is present, go to applicable test and repair.
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