Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Three-Way Catalytic Converter (TWC) Diagnostics
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 is reduced. The conversion capacity of the three-way catalytic converter (TWC) is reduced and unburned residue which is harmful to environment is released. To reduce the environmentally damaging emissions, the engine control module (ECM) checks the efficiency of the three-way catalytic converter (TWC). In brief, this check is carried out as follows.
To be able to check the catalytic converter, two heated oxygen sensors are used; one heated oxygen sensor in front of the catalytic converter in the engine compartment (front heated oxygen sensor) and a heated oxygen sensor after the catalytic converter in the engine compartment (rear heated oxygen sensor). After the rear heated oxygen sensor there is another catalytic converter, located at the front end of the propeller shaft tunnel under the vehicle's floor. However, this catalytic converter is not monitored.
The main function of the heated oxygen sensors 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 Ξ»=1. This mixture allows for optimum catalytic conversion.
On this version there are two banks on the engine's exhaust side, each one with catalytic converters and heated oxygen sensors according to the above description.
To evaluate the efficiency of the three-way catalytic converter (TWC) one uses a deviation added to the lambda signal.
This means that 8 to 15 minutes after the engine is started (depending on the market) catalytic converter diagnostics start.
The process is as follows:
A deviation is added to the lambda signal and the engine control module (ECM) is allowed to correct this.
The deviation switches between positive and negative values so that the fuel/air mixture switches between rich and lean.
A counter registers the number of switches occurring while the diagnostic is taking place.
If the rear heated oxygen sensor registers a large number of switches exceeding a parameter, a diagnostic trouble code (DTC) for three-way catalytic converter (TWC) efficiency will be stored.
If the parameter is not exceeded the three-way catalytic converter (TWC) is deemed to be operating well.
In a poorly operating way catalytic converter the switches caused by the deviation will quickly exceed the parameter. This is because the exhaust gases in principle pass straight through the catalytic converter without being cleaned. If the catalytic converter is operating well the switches will be slower as it takes a longer time for the variations to make themselves felt.
There are four catalytic converters, two for bank 1 and two for 2.
Bank 1; cylinder 1, 2, and 3.
Bank 2; cylinder 4, 5, and 6.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- β’ You smell fuel, burning insulation, or see smoke.
- β’ Brakes feel soft, pull hard to one side, or make grinding noises.
- β’ The engine overheats, stalls repeatedly, or misfires under load.
- β’ You are missing required tools, torque specs, or safe lifting equipment.
- β’ You are not confident in the next step or safety outcome.
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