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High Flow Catalytic Converters and the P0420 engine code

High Flow Catalytic Converters and the P0420 engine code

Dave10/5/2014
High Flow Catalytic Converters and the P0420 engine code One of the more popular modifications that we sell and install here at DDMWorks is a high flow catalytic converter. Although newer cars produced in the last 10 years have had more efficient and better flowing catalytic converters then in the past, aftermarket catalytic converters can typically still outflow them. This better exhaust flow can yield more power from the engine and also on turbo cars, the benefit of faster response from the turbo. 

As with most things, there is a trade off for this higher flowing exhaust. Typically when installing a higher flowing catalytic converter you will gain some extra exhaust volume, usually around about 15-20% louder without any other changes to the exhaust. The other issue to deal with is going to be a check engine light and a P0420 engine code - Catalyst efficiency below threshold. This code will keep the check engine light on and also cause you to fail emissions if you live in a location that checks them. Newer ECM's in cars are much smarter than they used to be and cars produced since 1996 that are OBD2 compliant, monitor the catalytic converter efficiency. All OBD2 compliant cars have 2 oxygen sensors installed in the exhaust, one sensor before the catalytic converter and one sensor after the catalytic converter. The job of the first oxygen sensor is to monitor the mixture of exhaust leaving the engine to make sure it is running the proper air/fuel ratio. This information is used by the ECM to trim fuel going to the engine to help make peak power at maximum efficiency. The output from the second oxygen sensor is compared to the reading from the first oxygen sensor by the ECM to make sure that the catalytic converter is functioning properly. If the ECM does not see the difference in the two oxygen sensors that it is programmed to see, then the ECM will cause the P0420 code to appear and a check engine light to appear on the dash. 

Since the high flow catalytic converters do not cause the same readings at the secondary oxygen sensor as stock, typically the ECM will trigger the P0420 code. This code does not necessarily mean that the high flow catalytic converter is not doing its job. Typically cars with a high flow catalytic converter will pass tailpipe emission testing, but the ECM will not see the readings it is programmed to see. Since we do not want to drive around with a check engine light on, there are a couple things we can do. 

One of the simplest things to do is install a oxygen sensor extender. This is a small metal tube that installs between the secondary oxygen sensor and the exhaust. It spaces the secondary oxygen sensor out of the flow of exhaust, which changes the signal it is sending to the ECM. This altered signal usually makes the ECM think that the catalytic converter is working like stock. In a very high percentage of the cases this will keep the check engine light off. We have these O2 extenders here at DDMWorks, or also a spark plug anti-fouler can be purchased from most auto part stores that will do the same thing. The next thing that can be done is to install a Oxygen sensor signal modifier. These are small electrical boxes that splice into the wires going to the oxygen sensor and modify the signal going to the ECM. These are not as common now since we have the ability now to modify the ECM directly. If the oxygen sensor extender does not keep the check engine light off, the code can be turned off in the ECM directly. One of the way we can do this is with the Trifecta tunes we offer. These tunes not only add power to your car, but also have the capability to turn off this P0420 code directly. Although Trifecta can not turn off the P0420 code themselves for legal reasons, they have made the ability to so available to the end user. When this code is turned off, the oxygen sensor is still active and the ECM still gets signals from it, however the ECM will no longer report it as an issue and no check engine light will come on. Adding a high flow catalytic converter is not typically one of the first engine modifications we suggest, but it can definitely be a nice way to open up the exhaust a little bit and make some more power. With the oxygen sensor extender or a ECM flash you will be able to have that extra power and response without the annoyance of a check engine light.

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