If you remove the cat I imagine the ecu would have been reprogrammed to make it ignore o2 sensors readings? If so would this be considered a remap?

When Changing the ecu in this way would it cause the check engine light not to come on in general anymore or will it still be working if it is running lean, rich etc for other reasons?

  • Aside: if you are in UK removal of the cat can mean MOT failure, unless the emissions are still in spec (unlikely). They use their own sensor, not the one in the car. Nov 13, 2020 at 16:48
  • If any visible cats are removed it's a straight MOT failure (under regardless of emissions, assuming the tester notices of course. Nov 16, 2020 at 15:53

2 Answers 2


You have misconceptions about how it all works together.

First, what @EᑎOT is accurate. The easiest way to deal with the ECU when the cat is removed is to put a dummy O2 sensor in place of a real one. If a "dummy" O2 sensor is in place of the original one (which actually works) it sends a constant signal back to the ECU in the "accepted range", which the ECU sees as normal, which won't set a CEL or throw a code.

Second, the downstream sensor which detects the function of the cat has nothing to do with how your car runs (NOTE: In newer cars, this isn't the case). It's just testing the efficiency of the cat (making sure it's working). If you remove the cat and "fix" it by changing the tune, the way it's done is by setting the parameters for which the ECU is reading from the O2 sensor so it never gets into bad territory. When the tuner does this, they "write" a new tune to the ECU. A "tune" in this case could generically be called a "remap", but the tuner could do this without even touching how the engine runs. The tuner would read the stock tune directly from the ECU, make the minor changes so the ECU thinks the cat is running within the "good" parameters, then write it back to the ECU. This takes specialized software and usually license tokens (for most aftermarket tuning software ... not sure about the Hondata stuff).

As @EᑎOT stated, it is far easier to just put a dummy O2 sensor in place than it is to put a new tune in the ECU. It's also a lot cheaper ... a LOT cheaper.


In my experience, when the car's catalytic converter is removed permanently, the sensors are replaced with a placebos, that respond (e.g. have certain resistance) in the way that the ECU expects when the converter is installed and functioning.

In most cases the above is easier than reprogramming the ECU.

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