XC70 L5-2.5L Turbo VIN 59 B5254T2 (2003)
The three-way catalytic converter (TWC) stores oxygen from the exhaust gases and uses it to make toxic gases more environmentally friendly. The
catalytic converter is a 3 way catalytic converter 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.
Three heated oxygen sensors (HO2S) check the common catalytic converter. One heated oxygen sensor (HO2S) is in front of the three-way catalytic
converter (TWC) for each bank of cylinders and a common rear heated oxygen sensor (HO2S) in the center of the three-way catalytic converter (TWC).
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 fuel / air mixture at around lambda=1. This mixture allows for optimum catalytic conversion. To determine catalytic converter
efficiency the amplitude of the heated oxygen sensor (HO2S) signals is compared (amplitude is a measure of signal oscillation).
When efficiency of the catalytic converter is good and the fuel / air mixture is normal, the front probe signals for banks 1 and 2 switch between rich and
lean while the rear probe signal is steady. If the three-way catalytic converter (TWC) efficiency is poor but the fuel / air mixture is normal, the rear probe
signal will also switch between rich and lean. This is because the exhaust gases flow straight through the three-way catalytic converter (TWC) without
being acted on. If the amplitude of the rear sensor becomes too great in relation to the amplitude of the front sensor for a number of checks, a diagnostic
trouble code (DTC) will be stored for catalytic converter efficiency.
