The Cat. Converter on my 2005 Ford Territory SUV (245 cubic inch Twin Cam 6 Cyl)Australian Model is stuffed and I'm removing it (they cost the National Debt 💸)....I know I can be arrested / fined a million dollars and possibly executed here for doing it, but another ain't going back on.

My question is : Will this cause the ECU/PCM/PCU or ECM thing to go crazy, or, as I was told by a wise man, "the above little box will just adapt itself and continue to tune the vehicle around the lack of Cat. Converter causing no real issues and maybe even marginally increasing performance, and consumption".

Your thoughts Ladies & Gentlemen ? (Keep 'em simple 'cause I ain't all that bright)

Cheers, The Village Idiot.😳

  • I removed the cat from my 1995 Mazda 2300b and at 1st it was sluggish and slow but soon began to run smoother faster and stronger and the more I drive it the more powerful it gets.
    – Jesse Wood
    Nov 12, 2020 at 19:03

5 Answers 5


Theoretically, the air / fuel mixture can run dry (to much air), which can cause serious damage to cylinder walls (sparks without enouth fuel = damage to cylinder). However, I think you need to remove more then one cat for that to happen.


When the ECU is in closed loop operation is uses the reading obtained from the lambda probe to adjust how much fuel is required by the engine. In order for this to work it has to measure the exhaust emissions before they enter the catalytic converter, were it taking a measurement after the cat, at the point where it recorded something that required a fuel trim, the mixture would have to be massively rich.

So, removing the cat should in no way affect closed loop operation. Some emissions control systems employ a second lambda probe after the catalytic converter as it is required by north american OBD / CAN laws. The signal from the probe after the cat is used purely to measure operation of the cat as the typical graph from this second probe ought to appear flat. In non-US applications, this second probe may not even be monitored by the ECU.

So, in summary, removing the catalytic converter from a modern fuel injected car shouldn't cause it any running problems but if it is fitted with a second probe after the cat, it may log a fault in the ECU relating to the operation of the catalytic converter only.


I’ve heard things about the cat providing back pressure and such, but in my experience a cat delete hasn’t revealed any issues to me other than smoking out the exhaust(which may not be the case with yours, the cat was absorbing the smoke in mine and it just revealed the problem when I removed it). TLDR, the delete shouldn’t majorly effect how the engine is running at all, so long there isn’t a sensor or something in the cat that feeds info to the ecu, then you’ll need to figure out what it does and bypass it!


I removed the cats on two old BMWs (a 2004 330Ci and a 1999 Z3, both with after-cat 02 probes), it increases hp and fuel econ and sounds amazing. The easy way to fool your ECU into thinking you still have cats are right angle pipes called defoulers, they mount to your exhaust pointing upwards and fool your sensors most of the time. the more advanced way is to recode your ECUs firmware so that the reading it wants of the secondary (after-cat) lamda probe is the one it gets, aka widen the acceptability range to include your maximum readings. You can use a recoding software like ECM Titanium but Im not going to get too specific because I haven't personally figured that out yet...working on it.


I removed the cat from a Honda as it had failed, I add oxygen supplements to the engine, so running with zero emissions, my ECU freaks every time it starts and have to reset it using an OBD scanner, when I removed the cat I installed a second silencer to maintain the exhaust gas pressure and reduce the lag, works brilliantly :)

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