P0420 & P0421 (Cat Efficiency) General Discussion

General discussion about tuning and related issues that do not apply to a specific brand of vehicle.
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P0420 & P0421 (Cat Efficiency) General Discussion

Post by hackish » Thu Jun 13, 2013 9:10 am

Tuners frequently encounter difficulties tuning for catalyst efficiency. This article will describe some of the common issues encountered as well as the testing methodology used by many ECUs to determine if a vehicle has a catalytic converter related fault.

Most modern cars have a 3 stage cat. These cats convert NO2, CO and HC emissions into N2, CO2 and water. The engine control computer uses oxygen sensors to control and monitor this reaction in order to ensure the tailpipe emissions are kept in check. Part of this control and monitoring strategy is to ensure the catalytic converter is working properly. When the engine controller determines that the catalytic converter is not working within its specified limits it will activate the check engine light, frequently with a P042x code indicating that the catalytic converter efficiency is below its threshold.

There are a number of complex tests performed to determine the catalytic converter efficiency. The first stage (reduction stage) of the cat breaks apart the oxides of nitrogen into nitrogen and oxygen. That oxygen and additional oxygen found in the exhaust stream is used in subsequent oxidation stages to turn CO into CO2 and HC into H2O and CO2. Since the catalytic converter uses up oxygen it is possible to monitor its functioning using the vehicle's oxygen sensors.

The front oxygen sensor and rear oxygen sensors are compared to see how much lag time there is and how much less oxygen is present after the cat as compared to before the cat. This is calculated into a ratio known as cat efficiency.

Failing oxygen sensors and failing catalytic converters are frequently symptoms of these codes (the system does actually work as designed). Those who have replaced their OEM cats with aftermarket "high flow" units will often see these codes as well. In general, most "high flow" cats really do have less efficient operation so again, the OEM system is in fact correctly reporting that the catalyst is not operating within specified parameters.

Vehicles with aftermarket header(s) can also cause problems because the cat is not able to reach and maintain a high enough temperature during cruise. The solution to this problem can be as simple as applying a heat wrap or having a ceramic thermal barrier applied to the headers.

There are a few scenarios where a tuner may wish to make changes to the catalytic control system to maintain the monitors with modified limits. Some vehicles have different equipment installed in different markets and need to be adjusted to match the market they are licensed in. As an example the Lotus Elise has a cat efficiency threshold ratio of 0.165 in the North American market but the limit is 0.300 in Europe. Replacing the ECU with a North American spec unit may subsequently expose the car to limits it is unable to pass. Race cars or off-road vehicles are another example where it may be legitimate and legal to change these limits.

In general, the higher the threshold the less effective the cat needs to be to pass the test. We highly discourage individuals from modifying these settings if it will violate your local laws but we leave it up to the individual to determine the legality of making these modifications.

As a final point, remember that it takes some time (often several driving cycles, warmup, drive, stop, cooldown) for the engine controller to complete its set of tests. The controller itself can often report the results numerically so the tuner can also see if the cat tests are close to passing or if the cat is completely ineffective.


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