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Help! I got a CEL/MIL with my Test Pipes Installed: P0420/0430/0139/0159

  • Posted: 01-10-2018 04:04 PM
  • Exhaust

Your street car is smart. It's looking for a catalytic converter and making sure it is working properly. It's actually required to by law. But your race car has cats deleted and you are getting a pesky check engine light because of it! Here is some info, and a couple rules of thumb to follow. In a V6 Honda or Acura, you may be using RV6 PCDs/HFPCs. In a 4-Cylinder Honda or Acura you may be using a Fastline Performance Track Pipe or other sort of cat delete. The codes are the same though, for either case. Read on for more!

First, there are typically sensors both before and after the main converters on the car. In Honda/Acura V6s that means there are four oxygen sensors because the main cats (primary converters) hanging off each bank of the engine.  The first, primary oxygen sensor, before the converter, is checking the exhaust gas coming out of the engine to feed back to the car's computer information that will help it run better. Yes, the main way the computer knows how to manage the engine is by monitoring the exhaust gas.

For more on the unique exhaust layout on V6 Honda and Acura models, read more here: An Outline Of The Exhaust Layout On J-Series Honda/Acura V6 Engines​.

The exhaust then passes through the catalytic converter to clean it up thereby reducing greenhouse gas emissions. After the converter there is another, secondary oxygen sensor. This sensor's only job is to make sure it sees cleaner exhaust than the first one. It doesn't have any impact on the running condition of the engine. The secondary sensor is only there to make sure the catalytic converter is working properly.

Of course, if you delete the cat, the secondary sensor is going to see the same exhaust readings in the first and second sensors and think something is wrong. This triggers a P0420 code, "Catalyst System Efficiency Below Threshold (Bank 1)." Bank one is the default because all engines have at least one "bank" of cylinders, but in a Honda V6 bank one means the one near the firewall. P0430 is bank 2, near the radiator. The codes literally mean that the secondary sensors are reading too little drop on emissions and the converters aren't working (or aren't there).

Now, it is important to note that the sensor is reporting a reading to the computer, and while the code is coming up because you deleted cats, it does not absolve the sensor for some blame. Sensors and circuits can go bad as well, so keep that in mind. If you didn't modify your converters and are getting these codes, it could be that your sensor(s) are just bad.

Back on topic. Cat-delete pipe providers offer provisions for off-road converter deletes called "defoulers." These extention-fittings are meant to space the secondary (not the primary) sensors out of the exhaust flow to ensure they are exposed to less emissions gases than the primary sensor to "fool" the computer into thinking there is still a converter in place. It's like...if the exhaust was a port-o-potty, moving the sensor 10 feet away instead of inside the closet. It smells less and isn't wigging out.

If you have a cat delete with a defouler installed and you are getting a P0420 and/or P0430, it's probably because the sensors are too close to the exhaust stream and the defoulers need to be adjusted out of the pipe a little. RV6 tries to preset these from the factory but it is impossible to get them right every time for every car. All the cars are a little different. Space them out and the clear the codes and they should not come back soon, or ever.

Now, think of the opposite situation...where the the secondary sensor might not read enough exhaust gas. Converters aren't perfect and the computer knows that. The secondary sensors need to read something toxic...if they don't the computer thinks maybe something is wrong with the sensors themselves. In these cases you will probably end up with a P0139 or P0135, again bank 1 or 2 for "O2 Sensor Circuit Slow Response." The translation here is "hey, these sensors aren't really reading much...check them out there could be a fault in the circuit." But what is really probably going on is the sensors are spaced out too much with the defoulers. You need to screw them in a little farther to trigger some readings to let the computer know they are working.

We hope this helps you cover the 90% of CEL issues you might have deleting the converters on your Honda race engine!

About the Author

Marcus di Sabella Marcus is the founder of Heeltoe Automotive. He's been working with cars (mostly Honda cars) since 1996, and has been providing enthusiasts with excellent products, services, and web experiences since 2002. He's been published in Honda Tuning, and holds a degree in Engineering Technology.


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