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Home >> Mazda >> 2002 >> Tribute LX, 4WD >> Repair and Diagnosis >> External Pages >> Different car >> Section 26 (Engine Controls - Self-Diagnostics - 2.0L) >> Diagnostic Tests >> DTC P0421: Catalyst Warm-Up Efficiency, Low >> Diagnosis & Repair Procedure

Diagnosis & Repair Procedure

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ensure FREEZE FRAME DATA has been recorded. Check for related service bulletins. Repair as necessary and go to next step.
  2. Check ignition timing. Check if vehicle excessively consumes oil or has been fueled using leaded gasoline. Check for engine misfire. Adjust or repair as necessary. If no problem is present, go to next step.
  3. Turn ignition on. Using scan tool, check for DTC P0300, P0301, P0303, P0304, P0117, P0118, P0125 or P1117. If any of these DTCs are present, go to appropriate DTC first. If DTCs are not present, go to next step.
  4. Using scan tool, access O2S11 (front HO2S) and O2S12 (rear HO2S) PIDs. Drive vehicle for 10 minutes at 40-60 MPH to ensure WU-TWC converter reaches operating temperature. Let engine idle and record PIDs for one minute. Scan tool should display graphs for O2S11 and O2S12. See Fig 1 and Fig 2 . Count number of times graphline crosses below .5V line (inversion) for both front and rear HO2Ss. Divide number of inversions for front HO2S by number of inversions for rear HO2S for ratio data. If ratio is 2 or more or no rear HO2S inversions, WU-TWC converter is functioning properly. See Fig 1 and Fig 2 . Replace suspect HO2S, then go to next step. If ratio is less than 2, WU-TWC converter has deteriorated. See Fig 1 and Fig 3 . Replace WU-TWC converter. After repair, go to next step.
    Fig 1: Identifying Front HO2S Graphline - Example
    G00143770Courtesy of MAZDA MOTORS CORP.
    Fig 2: Identifying Rear HO2S Graphlines - Examples For Properly Functioning WU-TWC Converter
    G00143771Courtesy of MAZDA MOTORS CORP.
    Fig 3: Identifying Rear HO2S Graphline - Example For Deteriorated WU-TWC Converter
    G00143773Courtesy of MAZDA MOTORS CORP.
  5. Check fuel pressure regulator vacuum hose for damage, and proper installation. Ensure vacuum is present at fuel pressure regulator vacuum hose. Repair as necessary. Reconnect fuel pressure regulator vacuum hose. Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Record fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. Record fuel pressure gauge reading. If fuel pressure is 31-46 psi (2.2-3.1 kg/cm2 ), go to next step. If fuel pressure is not 31-46 psi (2.2-3.1 kg/cm2 ), check fuel system. See SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. Repair as necessary.
  6. Check exhaust manifold, front exhaust pipe, rear exhaust pipe, muffler and tail pipe for cracks, loose connections, punctures, collapsed areas, dents or excessive bending. Repair or replace as necessary and go to step  11 . If exhaust system is okay, go to next step.
  7. Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and increase engine speed to 2000 RPM. Manifold vacuum should increase to greater than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, excessive exhaust system restriction may be present, go to step  9 .
  8. Leave tachometer and vacuum gauge connected. Ensure engine speed is at 2000 RPM. On a non-restricted system, manifold vacuum should quickly stabilize as RPM is maintained. On a restricted system, manifold vacuum will not quickly stabilize, or will slowly drop. If manifold vacuum is okay, should mean that no indication of exhaust restriction has been detected and testing is complete, or that catalyst is inactive and requires replacement. If manifold vacuum is low or slow to respond, moderate exhaust system restriction may be present, go to next step.
  9. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engine and increase engine speed to 2000 RPM and hold. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still drops or slow to stabilize, fault is in exhaust manifold or intake manifold gasket. To check for exhaust manifold restriction or casting flash, drop a length of chain into each exhaust port. Using wire or lamp to check ports may not indicate a small restriction. Repair or replace as necessary and go to step  11 .
  10. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and increase engine speed to 2000 RPM and hold. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and go to next step. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary and go to next step. Also check tailpipe/muffler assembly for debris from catalytic converter.
  11. Reconnect all connections. Turn ignition on. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine. Using scan tool, perform OBD II drive mode. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM. Using scan tool, access DIAGNOSIS MONITORING TEST RESULTS in GENERIC OBD II FUNCTIONS. Monitor TEST #10:10:11 (ratio of rear HO2S switches bank 1). If measured value is within specification, replace PCM and go to next step. See REMOVAL, OVERHAUL & INSTALLATION article. On vehicles equipped with immobilizer system, reprogram ignition key identification numbers. See COMPUTER RELEARN PROCEDURES - IMPORT article in GENERAL INFORMATION. If measured value is not within specification, go to next step.
  12. Perform DTC confirmation procedure. See DTC CONFIRMATION PROCEDURE  under SELF-DIAGNOSTIC SYSTEM. Check for DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If any other DTC is present, go to appropriate test.
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