Bought two aftermarket catalytic converters, codes P0420 and P0430

Hi Camp,

Both of these codes are saying the same thing. They are saying that the emissions measured by the o2 sensors is higher than the spec and it throws the code. This could be by three main reasons.

1. The converter is not reacting with the hydrocarbons in the exhaust and it is passing to much through to the atmosphere.
2. The downstream sensor is faulty and reading higher or more hydrocarbons then are there.
3. The upstream o2 sensor is reading lower or less hydrocarbons then what the downstream is reading.

The PCM is looking at the difference between the upstream and down stream sensor. When that difference is not correct then it sets the code for below threshold.

Assuming you have good parts that you replaced, I suspect you have an exhaust leak that is allowing air in and exhaust out. This is throwing off the measurement that these sensors are taking. They need to have a sealed system so that they can measure accurately.

Usually, the non welding converters come with a gasket and if you don't get them tight enough they will leak and you will have to most likely replace the gasket again and try to tighten them to ensure no leaks.

The easiest way you can tell if they are leaking is putting your hand around the joints/connection points of the exhaust with the engine running and see if you can feel exhaust pulses. You should not feel any pulses. Be careful not to touch the exhaust as it gets hot quickly.

Let me know what you find and we can go from there. Thanks

Okay. If you have no leaks and you did not have this issue before this happened, then there is something with the components you installed. These are two separate components with separate systems. Either you have an issue that is effecting both equally to set the same code on both sides, or the parts you have are faulty or not correct. I doubt the converters are incorrect especially if the sensors fit but I am wondering if they gave you incorrect o2 sensors. I have seen on other vehicles when you put upstream o2 sensors in downstream positions then it sets this code.

I do not know if the upstream (may be called Air Flow Sensors) are interchangeable with the downstream but clearly you have the same issue on both sides that you did not have prior. Before you go back to the parts store, just see if one of your downstream sensors will connect to the upstream connector on the wiring harness. If they do, then you may have the wrong sensors which would explain this.

If you have a scan tool that can read live data and not just a code reader, you can also look and see what the voltage is on all 4 sensors. If they are the incorrect downstream sensor, I suspect all the voltages will be just about the same. If they are, then the PCM thinks that the same amount of hydrocarbons before the converter are there after the converter and it sets an inefficiency code because it thinks the converter is not reducing them at all.

Does that make sense? Let me know what you find. Thanks

Yeah. Experience tells me if there were that many options that the ones you have are incorrect. I would suggest checking the build date. Even though your vehicle is a 2005 if it is an early build then more than likely it was built with 2004 parts. It is very typical for model years to bleed over when they have a parts change. Basically, they have left over inventory from the prior model year and just starting the new model year with the new style parts is a waste of money if they don't use the old style parts.

Your best bet is to call the local dealer, give them your VIN and get OEM part numbers for each of these parts. Then the parts store should be able to cross reference them into the correct ones that they are selling.

Let me know what you find but I am confident that this is your issue.