Cat coverters gutted: worse gas mileage?


1996 VDP 4 litre six.

I heard a horrible rumbling from the engine one day, and thought the crank was knocking.

Turns out the contents of the cat converters had broken up, and a golf ball size bit was knocking around in there. I had no choice but to remove the cat and knock the remaining bits out. With the O2 sensors attached, the engine has a rough idle, and the missfire code comes up. With the O2’s unplugged th engine runs great.
My local tech says that with the O2’s unplugged, the ECU reverts to a base map, but fuel mileage may increase.

Does anyone have any advise please?



I would agree that a base fueling map would come into play.

Whether or not this will improve fuel economy remains to be seen. It would depend on how well the feedback system was working before the O2 sensors were disabled.

Most typically you’d expect the feedback system, working properly, to improve fuel economy as the mixture is constantly being trimmed.


Did you swap the wires for the O2 sensors? It is easy to do if you haven’t makered them when you remove them and does bugger up the running. Try swapping them and see if that helps.

Peter and Doug, thank you for your quick responses.

Peter, the wire colours on the plugs and the harnesses don’t match at all. I’ve got red, yellow, white and black from the sensor, and a different mix on the car harness. There is no way to plug them in so both sides of the plug match in colour. I guess I need a diagram to be sure I have them correctly paired up. I’ll ask my local tech in the morning.

Doug, I made an error in my post. I meant to say that with the O2 sensors unplugged, and the ECU operating on its base map, that I likely will experience increased fuel consumption, not increased economy. As you say, with sensors working, they are always trimming for optimum.
Can you think of any long term issues with running with sensors unplugged?



From experience I can confirm fuel economy decreases when the cats are not in play, mine was a ‘92 XJ40 4.0 but it never had cats (= no o2 sensors) due to a moment of utter stupidity I fried the ECU and the only one I could find was for a cats car. Every time I started it there was a hesitation when the ECU looked for the feed back from the o2 and not seeing eny reverted to the base fuelling. This was on a car I was using for a charity run from Auckland to Bluff in the South Island so fuel economy of some sort would have been nice to have.

Don’t worry with the wire colours, just swap them and around and see if that improves things :slight_smile: