Catalyst System Efficiency Below Threshold

Code P0420 and 0430 indicates that the exhaust gas exiting the catalytic converter is too similar to the exhaust gas entering the converter. A faulty or overloaded converter due to an unbalanced air/fuel mixture can cause this condition.
Note the following contains a lot of acronyms. See the Glossary page for more information.

Monitor Execution:
The O2 sensors are used to monitor catalytic converter effeciency (see below). It is tested once per Driving Cycle. (On MIL [Malfunction Indicator Lamp] equipped vehicles this test must be failed three times before the MIL light is switched on.)
Failure is when emissions are estimated to exceed federal limits by 1.5 times.

How it works:
It seems complicated, but the system is designed so the fuel mixture actually oscillates between lean (too much air and not enough fuel) and rich (too much fuel), which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions.

If the converter is doing its job and is reducing the pollutants in the exhaust, the downstream oxygen sensor should show little activity (few lean-to-rich transitions, which are also called "crosscounts"). The sensor's voltage reading should also be fairly steady (not changing up or down), and average 0.45 volts or higher.

If the signal from the downstream oxygen sensor starts to mirror that from the upstream oxygen sensor(s), it means converter efficiency has dropped off and the converter isn't cleaning up the pollutants in the exhaust. The threshold for setting a diagnostic trouble code (DTC) and turning on the Malfunction Indicator Lamp (MIL) is when emissions are estimated to exceed federal limits by 1.5 times.

Monitor Sequence:
The following tests must be complete before the Catalyst Monitor test can run:

HO2S (Heated Oxygen Sensor) or O2 sensors monitor tests complete and no DTCs (Diagnostic Trouble Code)
No Fuel Trim DTCs
EGR (Exhaust Gas Recirculation) Monitor Complete
No Misfire DTCs
No SAIR (Secondary Air Pump) stuck on DTCs (where fitted.)

The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive.

  • Check for obvious problems like a disconnected wiring harness, which can be pulled off by driving thru a pile of snow or mud/rocks,
    or a shorted wire caused by rubbing on some other undercarriage part, common with lifted off-road vehicles.
  • Check the exhaust system for air leaks or holes rusted through. Any outside air that leaks into the exhaust can go through the cat as lean exhaust and hurt its efficiency.
  • Rap on the converter with a fist or a soft mallet and listen for anything loose or rattling inside. If the catalyst element is cracked or broken apart then the cat will need replaced.
  • You may want to test the O2 sensors for proper operation. Because these sensors tell the computer about the performance of the converter, malfunctioning O2 sensors can set this code. Verify that the sensors are faulty before replacing.
  • There may be another problem causing the engine to burn an excessively rich or lean air/fuel mixture. This could overload the cat and ruin its efficiency as well.
  • Common causes include incorrect fuel pressure, an EGR system malfunction, an MAF sensor malfunction, intake or vacuum leaks, incorrect cam timing, a bad PCV system, misfires, injector problems, cooling system problems, a restricted air filter, a head gasket leak, or a high oil level.

Some Forum comments:

The PCM (Powertrain Control Module) measures the switching rates of the upstream and the down stream 02 sensors. Because the cats store oxygen the downstream sensors switch at a slower rate. When they approach the rate of the upstream sensor the PCM sets a code. Yours is pending. Now a damaged sensor can also cause this. here are the most common causes as described by the manual; Use of leaded fuel, Malfunctioning ECT, High fuel pressure, Damaged exhaust manifold, Damaged catalytic converter, Oil contamination, Cylinder misfiring, Downstream wires improperly connected, Damaged exhaust system pipe, Damaged tailpipe assembly or Retarded spark timing. The cat would be high on my priority.

Install an O2 Simulator (Several companies make them) which replaces the O2 sensor (one site said it connects to the O2 sensor). You will eliminate the function of the O2 sensor which is to get the right air / fuel mixture.

Troubleshooting P0420 Catalyst Code at
O2 sensors

last updated 25 Mar 2010