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Home >> Mazda >> 2002 >> MX-5 Miata Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0420: Catalyst System Efficiency Below Threshold >> Diagnostic Procedure

Diagnostic Procedure

  1. Turn ignition off. Connect New Generation Star (NGS) tester or equivalent to OBD-II Data Link Connector-2 (DLC-2). See Figure. Turn ignition on. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check for service bulletins and/or other line of repair information related to this DTC. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  6. If repair information is not available, go to next step.
  2. Turn ignition off. Inspect front HO2S for proper installation. Start engine and check for exhaust leakage between exhaust manifold and front HO2S. If no problem is found, go to next step. If problem is found, reinstall and secure front HO2S and/or repair exhaust leak as necessary. After repair, go to step  6.
  3. Stop engine and leave ignition on. Monitor for DTC or pending coderelated to this DTC. If other DTC related to this DTCis present, follow appropriate DTC diagnostic procedure. If no other DTC is present, go to next step.
  4. Start engine and let it idle until it reaches normal operation temperature. Using NGS tester or equivalent, access O2S11 PID. Verify O2S11 PID while racing engine (in PARK). Ensure that each value 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 front H2OS. After repair, go to step  6.

  5. 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 - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. If problem is found, replace TWC and go to next step. If no problem is found, replace rear HO2S and go to next step.
  6. Return vehicle to original condition. Using NGS tester or equivalent, clear DTCs from PCM memory. Start engine and run DRIVE MODE 1, 2 and 3. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM.Monitor for pending DTCs. 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. If any other DTC is present, follow appropriate DTC diagnostic procedure.
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