P0420

Circuit Description
To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-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, converting the HC and CO into harmless water vapor and carbon dioxide. The catalyst also converts NOx, to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalyst's ability to control emissions. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the 3-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst is considered to be deficient and the MIL is illuminated if subsequent tests also indicate a malfunction.

Conditions for Running the DTC
1. The conditions for engine warm up are met. Use the scan tool Catalyst Data List in order to verify the following:
^ None of the following DTCs are set:
- Throttle position (TP) sensor DTCs
- Vehicle speed sensor (VSS) DTCs
- Intake air temperature (IAT) sensor DTCs
- Manifold absolute pressure (MAP) sensor DTCs
- Exhaust gas recirculation (EGR) pintle position DTCs
- Heated oxygen sensor (HO2S) sensor DTCs
- Idle air control (IAC) valve DTCs
- Crankshaft position (CKP) sensor DTCs
- Engine coolant temperature (ECT) sensor DTCs
- Crankshaft position (CKP) sensor DTCs
- Mass air flow (MAF) sensor DTCs
- Idle speed DTCs
- Misfire DTCs
- Fuel injector fault DTCs
^ The engine is running for more than 10 minutes.
^ The ECT is more than 76°C (169°F) and less than 124°C (255°F).
^ The BARO is above 75 kPa.
^ The vehicle is in closed loop.
^ The IAT is more than -20°C (-4°F) and less than 100°C (212°F).
^ Fuel trim DTCs
^ The battery voltage is more than 10.7 volts
2. Warm up the catalyst.
^ Fully open the hood.
^ Place the transmission in park (automatic) or in neutral (manual).
^ Set the parking brake.
^ Press and hold the service brake.
^ Each time the engine is started the diagnostic may run up to 18 times. After the 10 minute run time and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1500 and 2500 RPM. For any additional tests on the same key cycle, the engine speed must be between 1500 RPM and 2500 RPM for 1 minute.
^ To activate the diagnostic, return to idle and put the vehicle in drive (automatic) or depress the clutch (manual).
3. Test the catalyst.
^ Verify that the transmission is in drive (automatic) or in neutral with the clutch depressed (manual).
^ Within 60 seconds the air fuel ratio goes rich below 14.1 for up to 7 seconds, then may go lean above 15.3 for up to 10 seconds.
^ Verify that DTC P0420 has passed or failed this key cycle using the scan tool.

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) during the first trip in which the diagnostic test has run and failed.
^ The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed.
^ The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids
Inspect or test for the following conditions:
^ The PCM will not enable the catalyst test until following conditions are met:
- The engine idle speed is plus or minus 100 RPM from the desired idle.
- The Throttle position (TP) is 1% or less.
- The short term integrator is between -20% and +20%.
^ The catalyst test aborts if the vehicle falls outside the conditions listed below while the test is running:
- The engine idle speed is plus or minus 125 RPM from the desired idle.
- The Throttle position (TP) is 1% or less.
- The 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). If this problem occurs, use the scan tool in order to force the cooling fans on return to Step 2 in the table.

Important: When using the scan tool to force the cooling fans on, use the Next list and Previous list soft keys in order to enter the Catalyst Data list. If the Catalyst Data list is not entered this way, the cooling fan control is canceled.

^ The number of attempted tests is limited to 18 per key cycle.
^ More than 6 tests may have to be attempted in order to get 6 completed tests. An aborted test counts as an attempted test.
^ If 18 tests are attempted and a determination is not made this key cycle, turn the key off for 30 seconds. Start the vehicle and perform the Conditions for Running the DTC, including the 10 minute engine run time.
^ After returning to an idle, the HO2S 1 signal may stay rich or lean for several seconds causing the test to be delayed.
^ Inspect for a faulty connection at PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
^ Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving the connectors and wiring harnesses related to the sensor. A change in the display indicates the location of the fault.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
1. The Diagnostic system check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable.
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 tested up to 18 times and completed up to 6 times this ignition cycle. If the A/C is not turned off the diagnostic may not run. 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.
9. 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, leaking or plugged fuel injectors, high engine oil or coolant consumption, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.