Catalytic converter diagnostic

The three-way catalytic converter (TWC) stores oxygen found in the exhaust gases and uses it to make toxic gases more environmentally friendly. The catalytic converter is a three-way catalytic converter (TWC) in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx (nitrous oxide) is reduced. As the three-way catalytic converter (TWC) ages its ability to store oxygen drops. This reduces the conversion capacity of the three-way catalytic converter (TWC). To avoid dangerous emissions the engine control module (ECM) checks three-way catalytic converter (TWC) efficiency. In brief, this check is carried out as follows.

Two heated oxygen sensors (HO2S) are used to check the catalytic converter; one upstream and one downstream of the catalytic converter. The main function of the heated oxygen sensors (HO2S) is to measure the oxygen content in the exhaust gases so that the engine control module (ECM) can maintain the engine fuel/air mixture at around λ=1.

The catalytic converter diagnostic consists of two checks. The first checks how quickly the three-way catalytic converter (TWC) begins to operate, and the second is a response time check. When starting cold the control module checks how quickly the three-way catalytic converter (TWC) begins to operate. If the target lambda is already varying during this check, the following check is not carried out.

When the engine is at operating temperature, lambda variations are created (Dither function) by the diagnostic, adding a deviation to the target lambda signal which the fuel control system regulates towards. The deviation changes between positive and negative values so that the fuel/air mixture changes between rich and lean. The discharge through the three-way catalytic converter (TWC) is determined by studying the variation of lambda and the response from the rear probe.

On a poorly performing three-way catalytic converter (TWC), the deviation changes can easily be calculated by the rear probe response time. On an efficient three-way catalytic converter (TWC) this is more difficult, because the variations take too long to go through.