Does a CEL being active matter for mpg?
 

So back track three months and I had a full exhaust system installed (long tube headers, custom fabricated mid pipe, dual pipes side flared) with a mid pipe that was hand build to fit my truck that included 3 high flow cats (one main, two pre-cats). Anyway once winter hit with colder air my engine has been intermittently throwing the P 0420 CEL code from time to time. Does this effect open/closed loop operation? Hurt MPG?

Probably not. If upstream sensor is fine, then air/fuel should be regulated properly.

if you're concerned, you could use exhaust wrap or a type of insulation to keep the exhaust hotter. My guess is you're getting a lot of air blowing through the engine bay and on the exhaust cooling it too much.

If the P0420 is caused by a leak or the fuel injectors are not performing within spec, then it could lead to poor fuel economy, but if it's actually the cat performance that has deteriorated, it won't affect fuel economy.

I've punched out the honeycomb out of several cats when they physically failed, and then just ignored the p0420 CEL. Some people put a spacer in downstream O2 sensor to fool the computer into thinking everything is OK.

This has the best info I've stumbled across.

So I should be fine if I get something to fool the computer?

get this http://www.summitracing.com/parts/vp...PdGRoCJgbw_wcB
or this for less Nyppd O2 Oxygen Sensor 2 in Extender Spacer Dry Cell H2O HHO Wideband 18mm 7 8 S | eBay

Hopefully it doesn't send the car into limp mode... I cant imagine that's good for economy.

-soD

My car can't go into a true limp mode, too many mechanical parts and not enough electronic controls for it to really start restricting things. Worst thing it can do is that it will resort to WOT fuel maps to prevent engine damage.

Thank ya kindly, ordered one

CEL can lower fuel economy because the light will cause just a tad bit more strain on the alternator. If nothing else is wrong, then you should be fine, since the extra strain of that one small light being on would be equivalent to 0.000001 MPG or so.