Car used to get 20 city, 23+ highway. Then started getting about 17 city, and 18-19 highway. I had a P0420 a few weeks ago, changed both 02s (Bosch) as my car has 156 and figured they were due for changing. About 700 miles and CEL came on, codes are both P0430 and P0420, indicating poorly functioning cat. Odds are I need to change the cat, BUT I figured Id check here and see if any other possibilities exist to throw these codes? Also, would a bad cat lower milage? My car feels fine, smooth idle, still pulls strong when I get on it, so doubt cat is clogged?

 

In a MPFI car having a bad cat will strongly affect MPGs as the O2 Sensors cant give good readings to the ECU.
BUT You already had a terrible performance with 23mph on Highway. 20 on the city isn't bad but I don't know why you have such a bad MPG in the Hwy usually is the other way around...

 

Try installing the OEM brand of 02 sensors. The Bosch are not recommended.

 

Hve you actually heard of anyone ever having problems using Bosch O2 sensors?

 

^ Yes.. Use DENSO Only.

 

So, does the car have any other apparent issues besides poor gas mileage ?
When was the last tune-up ? ( plugs, filters, etc.) Try a fuel system cleaner.

P0420 = Catalytic System Efficiency below threshold.
Try clearing codes and see if they come back. Could be a faulty #2 secondary 02 sensor.

A badly clogged cat would cause serious runabilty issues and you would know.

 

Tap on the cat and see if you can hear anything that sounds loose. If you replaced both O2s and the cel came back with those 2 codes its a good possibility that the cat is bad. And yes, in the past the Bosch O2s werent recommended for our cars and often failed quickly.

 

reminder to stay away from Bosh spark plugs-- in the TL-- (fine in Bimmers!)
they burn out quickly and cause problems with coil system

denso was OE part for O2 sensor and known to work well with all related parts

where I live muffler shops will run a few test- infrared temp scan, pressure test etc on cat for no or low cost.

Do you smell rotten eggs after a high rpm run? = cat clog
low response with full throttle jump at 4500 and up can be clog cat

on fuel cleaners- seafoam in the gas tank cleans O2 sensors and the cat
wont fix dead parts but is good preventative maintenance

 

pls verify you reset the ECU by removing the CLOCK fuse or battery cable?

and it has NGK spark plugs? the 8 dollars each ones?