Catalyst performance below threshold

P0420

• 4-sensor: rear (RH) bank manifold catalyst
• 3-sensor: main catalyst

P0421 - rear (RH) bank manifold catalyst

P0430 - front (LH) bank manifold catalyst

P0431 - front (LH) bank manifold catalyst

Emissions Equipment Information

Three o² sensor system (KL engine with one catalytic converter) - A front sensor is positioned on the LH bank and a middle sensor is positioned on the RH bank upstream of the catalyst for closed loop fuel control.  A rear sensor is mounted downstream of the catalyst to monitor its efficiency.

Four o² sensor system (KJ / KL engine with three catalytic converters) - Front sensors are mounted upstream of the RH and LH warm-up three-way catalyst (WU-TWC) for closed loop fuel control.  Rear sensors are mounted downstream of the WU-TWCs to monitor their efficiency.

Detection Conditions

• The ECU monitors the inversion frequency of the oxygen sensors and compares the number of pre-catalyst and post-catalyst sensor inversions to calculate the inversion ratio.  If the inversion ratio is below the preprogrammed threshold, the ECU determines that the catalyst system has deteriorated.

  Monitoring Conditions:

  • engine speed is between 1250-2500 rpm
  • calculated engine load is between [KL 16-50%] [KJ 20-50%]
  • vehicle speed is between 24-75 mph
  • coolant temperature is above 80 degrees C.

Diagnostic Support Notes

• This is an intermittent monitor (CATALYST)
• Check engine indicator illuminates if the ECU detects the above malfunction condition in two consecutive drive cycle
• DIAGNOSTIC MONITORING TEST RESULTS available
• PENDING CODE is not available
• FREEZE FRAME DATA is available
• DTC is stored in ECU memory

Possible Causes

• Catalytic converter deterioration or malfunction
• Exhaust gas leakage
• Oxygen sensor not tightened properly
• Oxygen sensor malfunction

Additional Information

See http://tsb.milleniatech.net for information on TSB Item Number 01-003/01 regarding overly sensitive rear o2 sensors erroneously storing these codes on some Miller Cycle engine equipped models.

[P0420 / P0421 / P0430 / P0431 - From 1998 and 2002 Mazda Millenia factory service manuals]