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

An Intermittent problem may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation.

Any circuitry, that is suspected as causing the intermittent complaint, should be thoroughly checked for the following conditions:

Refer to Intermittent Conditions .

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD 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. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.

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 it's failure.

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

5. If the three-way catalytic converter needs to be replaced, make sure that another condition is not present which would 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.