I have an 03 max and its been a great car but noticed a huge drop in mileage (by approx 200km less to the tank than in the past.)

Here is the background.

Gutted two pre cats and enede up getting a code for p0420 so i ended up putting in Two Non Foulers / 02 extender to get rid of the problem.

The SES light did not come back but i ended up getting P0159 code showing a Slow response for that same sensor. I cleared the code and put on approx 400km and it hasnt come back but i noticed my fuel mileage has gotten worse. My DTE uses to say 600km and now only 498 and i have to fill up more often now and get only 400 km per tank..

Wondering if anyone is having the same issue after installing Non Foulers?

Any ideas would be helpful..

 

I think a tune could solve the ses, but gutting the cats could be your mileage problem.

 

I know if the primary sensor goes out, then the ecu pulls trading from the secondaries. But, I'd imagine that you would have a primary code if that were the case.

Has there been anything else that's changed? Got fuel somewhere different? Verified you have kept proper air pressure in tires with colder weather setting in? Have you verified your rear brake calipers haven't got a stuck e-brake line holding pressure on the brakes? Just a couple thoughts of some areas to check out.

 

what makes you say that, please explain

IIRC gutting precats shouldn't effect mileage at all (that's if you install O2 Sims to the bungs)

 

gutting my rear precat did not affect anything, i also have a non fouler installed in the o2 bung.