QOTW #1 – Do upgraded catalysts cause emissions faults?

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Welcome to the section of BamfordRose channel where we look at some questions that we can see in online places, forums, social media, that sort of thing. I’ve sent one question where a full member is asking, “I’ve got upgraded catalysts on my car yet my car is failing the MOT. Is this normal?” (The short answer is) It’s not normal. It shouldn’t happen. If we talk about the standard catalysts, the upgraded catalysts and what you need to do in the process of fitting the catalyst, it should all become understandable.

On the bench here, we’ve got a standard factory catalyst and an upgraded to 200 cell catalyst. This actually is two catalysts within one cane. There’s a catalyst here, an air gap and a catalyst here. The performance upgrade comes from reducing the volume of the catalyst, allowing on the high flow gap. There is more air to flow through and less exhaust back pressure, so that’s where the performance increase comes from.

When someone fits a 200 cell cat, it replaces the two sections. I think this is a 900 cell per square inch, this is a 400 square inch, whereas this is 200 cells per square inch, so these are the 200 cell honeycomb. When you hear 200 cell cats, it’s 200 cells of a square image. It’s obviously going from 900 and a 600 catalyst to a 200 cell catalyst is much higher airflow, therefore, more performance from this part.

If you just get some of these through the post and fit them to your car, then the lambda sensors that are measuring the exhaust gas coming out the engine… This is pre catalyst, this is post catalyst and between the two of them, it looks for gas conversion and assesses that a certain amount of gas conversion is taking place.

If we now go to a 200 cell catalyst and a sensor measures gas flow pre- and post-catalyst because there is less gas conversion going on within this catalyst, then the difference between the two sensors is going to be less than what it was on these sensors. If you just fit these upgrade catalysts and then drive and the system flashes an emission service light on the dashboard, it’s most likely because on the upgraded catalyst, the conversion is less. It doesn’t meet the standard; therefore, the system thinks of its error and puts the emissions lights on.

So, the answer to this question is really you should never fit an upgraded catalyst without an ECU remap. This is because the ECU remap is going to change in spark and fueling to get yet more performance from the catalyst upgrade because this is going to run cooler than this catalyst, so you can optimize spark and fueling to make this run hotter and then get even more power. The ECU remap should be because you’re going to get more performance which is the whole idea of fitting an upgraded cat to start with.

But the next, the ECU remap should readjust the diagnostic, so that the upgraded catalyst which ultimately converts less gas no longer is sensitive to the lower conversion and puts that error light on the dashboard.

This is the reason why if you just have fit catalysts without an ECU remap, you’re most likely going to get error lights on the dashboard. So, remap the ECU to avoid that.

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