Ive read a ton of posts on this issue about the p0240 code and have an interesting issue with this code. I understand that the p0240 code basically means that the cat is bad however i haven't had a cat in my 8 for almost 2 years and never had a single problem with it.

Lately however the car has been somewhat hard starting and running kinda awkwardly it seems to be either way too rich or too lean at any given point. it started running very poorly in this respect and severely dropped gas mileage then a code appeared showing the p0240 catalyst system efficiency below threshold (bank 1).

Based on this i would assume that i am needing a new upstream o2 sensor however i haven't been able to find any info at all either way. With the price of this o2 sensor im leary to jump into it and looking for any input i can find.

I have recently done coils plugs and wires. oil looks perfect everything seems to be in normal operation but the code pops once a week now and mpg jumps drastically day to day (8 one day 22 the next).

 

Right ... I'm guessing it's an 04

Might be handy to indicate
- What year?
- Miles?
- Model?
- Coils last replaced
- Spark plugs last replaced
- other maintenance, like a fuel pump
- Modifications like a Cold air intake

Sounds like a Front O2 sensor ... no way know unless you monitor the AFR's.

 

sorry should have included that in original message forgot to add that info.

2005 with 62000 miles. 6 speed manual transmission. Spark plugs were replaced last month when performance started to be a bit awkward.

Replaced coils with MSD coil mod at 45000 due to a coil failure which initially took out the catalytic converter. odd part of this was that the o2 sensors were both pre-cat leaving no issues at all with bypassing.

Haven't had any other issues at all throughout the duration of owning the car.

And yes guarantee i voided the warranty by bypassing catalytic converter but the nearest dealer is about 400 miles away from me making it very hard to take it to the shop for problems.

 

well it can only be two things. the cat or the sensor and you dont have one of those....