The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx into nitrogen. The PCM has the capability to monitor this process using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. The capacity in turn indicates the catalyst's ability to the convert exhaust emissions effectively. Whenever the catalyst functions correctly, the HO2S signal will be far less active than the signal produced by the front Oxygen Sensor (O2S). The PCM monitors the catalyst efficiency in 2 stages. When the catalyst efficiency falls below 75 %, the PCM sets a stage 1 fail flag but will not illuminate the Malfunction Indicator Lamp (MIL) at this point. When the catalyst efficiency falls below 50 %, the PCM will set a stage 2 fail flag, and the MIL will illuminate.

The numbers below refer to the step numbers on the diagnostic table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the Freeze Frame Data and the Failure Records on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurs. Refer to this information on the scan tool later.

2. Whenever component DTCs are set, diagnose those DTCs first. A fault in a component may cause the three way catalytic converter to appear degraded or may cause the converters to fail.

3. This checks for conditions that cause the three way catalytic converter to appear degraded. Repair any problems found before proceeding with this Table.

5. Whenever the three way catalytic converter needs replacement, make sure that another condition is not present which caused the converter to become damaged. These conditions may include: misfire; high engine oil or coolant consumption; retarded spark timing or weak spark. To avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.