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HOMESERVICE MANUALSMAZDA1999PROTEGE ES, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROLS - SELF-DIAGNOSTICS - 1.8LDIAGNOSTIC TESTSDTC P0421: WARM-UP CATALYST SYSTEM EFFICIENCY BELOW LIMIT (CALIFORNIA EMISSIONS)DIAGNOSIS & REPAIR PROCEDURE
1999 Mazda Protege ES, Automatic

Diagnosis & Repair Procedure

1999 Mazda Protege ES, AutomaticSECTION Diagnosis & Repair Procedure
  1. Ensure FREEZE FRAME PID DATA and DIAGNOSTIC MONITORING TEST RESULTS have been recorded. Verify stored DTCs. If DTC P0102, P0103, P0122, P0123, P0500, P1102, P1103, P1122 and/or P1123 are present, inspect and repair first, then go to step  11. If listed DTC are not present, go to next step.
  2. Determine if DTC P0420 is recorded on FREEZE FRAME PID DATA. If DTC P0421 is present, go to next step. If DTC P0421 is not present, inspect and repair any other recorded DTCs. Go to step  11.
  3. Erase DTCs. See CLEARING CODES under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION - PROTEGE 1.6L & 1.8L article. Conduct drive cycle mode No. 3 under DRIVE CYCLE PROCEDURE. Erase DTCs. See CLEARING CODES under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION - PROTEGE 1.6L & 1.8L article. Using scan tool, verify TEST #10:11:11 in DIAGNOSTIC MONITORING TEST RESULTS. If test exceeds MIN value, go to next step. If test does not exceed MIN value, problem may be intermittent. Ensure all applicable connections are clean and tight. Recheck for DTCs.
  4. Using scan tool, access PID/DATA MONITOR. Record PID data for MAFV, TPV and VS. Compare readings with specifications. See PIN VOLTAGE/PID VALUE CHARTS article. If any data is not within specification, go to applicable test in SYSTEM & COMPONENT TESTING - EXCEPT B2500, B3000 & B4000 article. If all data is within specification, go to next step.
  5. Referring to FREEZE FRAME PID DATA, operate vehicle under same conditions as when DTC was set. Record PID data for MAFV, TPV and VS. Compare readings with specifications. See PIN VOLTAGE/PID VALUE CHARTS article. If any data is not within specification, go to applicable test in SYSTEM & COMPONENT TESTING - EXCEPT B2500, B3000 & B4000 article. If all data is within specification, go to next step.
  6. Inspect exhaust system for any leakage. Repair as needed. Go to step  11. If exhaust system is okay, go to next step.
  7. Inspect front and rear heated oxygen sensors. See SYSTEM & COMPONENT TESTING - EXCEPT B2500, B3000 & B4000 article. Replace as needed. Go to step  11. If both heated oxygen sensors are okay, go to next step.
  8. Inspect front heated oxygen sensor heater. Repair as needed and go to step  11. If heated oxygen sensor heater is functioning correctly, go to next step.
  9. Inspect rear heated oxygen sensor heater. Repair as needed and go to step  11. If heated oxygen sensor heater is functioning correctly, replace catalytic converter. Go to next step.
  10. Repeat step  3. If test exceeds MIN value, go to next step. If test does not exceed MIN value, go to step  2.
  11. Using scan tool, verify PENDING TROUBLE CODE DTC and DIAGNOSTIC MONITORING TEST RESULTS. If any DTCs are present, go to applicable test and repair as needed. If no DTCs are present, testing is complete.
RENDER: 1.0x

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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.