Diagnostic Overview - Catalyst System

The PCM makes a determination of the Catalyst efficiency based on the comparison of the Upstream and Downstream O2 Sensors. Besides an obvious failed catalyst, there are other conditions that can cause the Catalyst Monitor to fail. These conditions should be considered and checked before replacing a Catalytic Converter.

• A leak in the exhaust system, especially before the Catalytic Converter can cause an the Upstream O2 Sensor to drift out of range or not switch properly and fail the Catalyst Monitor. Depending on the location of the leak, even a very small exhaust leak can cause the Catalyst Monitor to fail.
• The catalyst protects the Downstream O2 Sensor from fouling but the Upstream O2 Sensor is located directly in the output path of the cylinders and is not protected. An engine that has a mechanical failure causing it to burn oil, coolant or excessive fuel can foul the Upstream O2 Sensor and cause the Catalyst Monitor to fail.
• A new Downstream O2 Sensor paired with an aging Upstream O2 Sensor can sometimes cause the Catalyst Monitor to fail.