2000 Mazda B2500 SX, AutomaticFACTORY OEM SERVICE PROCEDURE

Catalyst Efficiency Monitor Malfunction

DTC P0420, P0430 DETECTION CONDITION AND POSSIBLE CAUSE

DTC P0420, P0430 CATALYST EFFICIENCY MONITOR MALFUNCTION
DETECTION CONDITION 
  • P0420 indicates Bank 1 catalyst system efficiency is below acceptable threshold.
  • P0430 indicates Bank 2 catalyst system efficiency is below acceptable threshold.
POSSIBLE CAUSE 
  • Use of leaded fuel.
  • Oil contamination.
  • Cylinder misfiring.
  • Damaged HO2S.
  • Malfunctioning ECT sensor.
  • Downstream HO2S wires improperly connected.
  • High fuel pressure.
  • Damaged exhaust system pipe.
  • Damaged exhaust manifold.
  • Damaged muffler/tailpipe assembly.
  • Damaged catalytic converter.
  • Retarded spark timing.
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, such as misfiring.
  • Incorrect HO2S signal input (e. g., rich/lean signal when engine is operating under rich/lean conditions) can cause an abnormal temperature increase in the catalyst.
  • ECT sensor DTCs can indicate that the thermostat is not operating correctly or that the engine coolant level is not filled to specification, producing above normal operating temperatures.
HINT 
G03988135Courtesy of MAZDA MOTORS CORP.
DTC P0420, P0430 INSPECTION

STEP INSPECTION ACTION
1
NOTE: If entering this test from the symptom troubleshooting, go to next step.
  • Was PCM Quick Test performed? 
 Yes If any other DTC is recorded, go to appropriate DTC troubleshooting first. If only DTC P0420 and/or P0430 are recorded, go to next step.
No Perform PCM Quick Test.
2 CHECK REAR HO2S WIRING AND PCM CONNECTIONS
NOTE: If the electrical connections of the rear HO2S are interchanged/crossed, the Catalyst Efficiency Monitor Test will fail.
  • Inspect the wiring of the rear HO2S for proper routing and connection.
  • Disconnect the PCM and inspect for damaged/pushed out pins, corrosion and loose wires.
  • Are there any concerns with the HO2S wiring or the PCM connection? 
 Yes Repair any wiring or connection concerns. For PCM pin concerns, replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to step 10.
No No Engine Control System related to DTCs or symptoms.
Go to next step.
3 CHECK FUEL PRESSURE
WARNING: The fuel system will remain pressurized when the engine is not running. To prevent injury or fire, use caution when working on the fuel system.
Factory Service Tools

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.

Shop Manual-Ready ToolsCompare Repair QuoteQuote Shield Pro

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.

Rather have a pro handle this?

A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.

Comes to youSame-day available12-month warranty
Get a Free Quote

No obligation. See pricing before you book.