P0420

Circuit Description
In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and carbon monoxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM monitors this process via the post catalyst HO2S signal. The post catalyst HO2S, located in the exhaust stream after the three-way catalytic convener, produces an output signal which indicates the oxygen storage capacity of the catalyst. Oxygen storage capacity is one method of determining the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the post catalyst HO2S signal will be far less active than that produced by the pre-catalyst HO2S. If the PCM detects excessive post catalyst signal activity (outside of an acceptable range) for an extended period of time, the PCM will set this DTC. This indicates that the three-way catalytic convertor's oxygen storage capacity is below a threshold considered acceptable.

Conditions for Running the DTC
1. Meet conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following.
^ No misfire, Fuel Trim, Injector or ignition control circuit, EVAP, HO2S, VSS, IAC, EGR,TP, IAT, MAP, ECT, MAF, or CKP DTCs set.
^ Engine has been running longer than approximately 9 minutes.
^ IAT is between -18°C and 80°C (0°F and 176°F).
^ ECT is between 75°C and 120°C (167°F and 248°F).
^ Baro is above 75 kPa.
^ Closed Loop.
2. Warm up the catalyst.
^ Fully open hood.
^ Transmission is in park (automatic) or neutral (manual).
^ Set the parking brake.
^ Engine speed is greater than 1500 RPM for 1 minute.
^ Return to Idle.
3. Test the catalyst.
^ Transmission is in drive (automatic) or neutral (manual).
^ With in 5-20 seconds HO2S 1 should toggle rich above 600 mv for up to 7 seconds, then it may toggle lean below 300 mv for up to 7 seconds.
^ Using the scan tool check and see if
DTC P0420 has passed or failed this ignition cycle

Conditions for Setting the DTC
The PCM determines that the catalyst's oxygen storage capacity is below a threshold considered acceptable.

Action Taken When the DTC Sets
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL Off after three consecutive drive trips that the diagnostic runs and does not fail.
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A History DTC clears after forty consecutive warm-up cycles, if this or any Other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Diagnostic Aids

Notice: Use the Connector Test Adapter Kit J 35616-A for any test that requires probing the following items:
^ The PCM harness connectors
^ The electrical centre fuse/relay cavities
^ The component terminals
^ The component harness connector
Using this kit will prevent damage caused by the improper probing of connector terminals.
Check for the following conditions:
^ The PCM will not enable the catalyst test until following conditions are met:
- Engine speed is plus or minus 100 RPM from desired idle.
- Throttle Position (TP) is 0%.
- Short term integrator is between -20% and +20%.
^ The catalyst test will abort if the vehicle falls outside the conditions listed below while the test is running:
- Engine speed is plus or minus 125 RPM from desired idle.
- Throttle Position (TP) is 0%.
- Short term integrator is between -20% and +20%.
^ The catalyst test may abort due to a change in engine load (i.e., A/C, coolant fan). Repeat the test if a load change occurs during the test.

^ More than 6 tests may have to be attempted to get 6 completed tests. An aborted test counts as an attempted test.
^ The number of attempt tests is limited to 18 per ignition cycle.
^ After returning to an idle the HO2S 1 signal may stay rich or lean for several seconds causing the test to be delayed.
^ Poor connections at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
^ Harness Damage. Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
2. If any component DTC's are set, diagnose those DTC's first. A fault in a component can cause the converter to appear degraded or may have caused its failure.
3. Clearing the DTC's allows the catalyst test to be completed up to 6 times this ignition cycle. The diagnostic will abort if the A/C clutch cycles. Turn the A/C off. The engine must be warmed-up. The converter needs to be warmed-up by raising the engine speed above idle for the specified time prior to each attempted test. Check and see if DTC passed or failed this ignition cycle. If the DTC does not pass or fail look for a possible reason that would cause the test to abort.
4. This step includes checks for conditions that can cause the three-way converter to appear degraded. Repair any problems found before proceeding with this table.
7. If the three-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include: misfire, high engine oil or coolant consumption, HO2S silicon contamination, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.