just recently put a flowmaster super44 on my jeep and yesterday the check engine light came on and i got the computer code 12-21-55. meaning my o2 sensor is bad. could this be because of the new muffler?

thanks,
joma

 

If your '96 has just the upstream(pre-cat) sensor, I'd eyeball it to make sure it wasn't just knocked loose or damaged during your muffler install. If not, then I'd replace it.. how many miles on the current one? A lot i'll wager.
I just replaced the 160,000+ O2 sensor on my '93 and it made a noticeable improvement from the very first startup. The idle, which before jumped up and down from 745 to 755 rpm (approx.) now sticks pegged on 750. But hey! we ain't talkin' about mine.. we's talkin' about yourn I always get off subject..
My thinkin' is.. it tripped the O2 code, it wants a new one. Give the Jeep what it wants... "resistance is futile".. "It puts the lotion on its' skin, or else it gets the hose again"

 

The muffler has nothing to do with o2 sensor readings. In a nutshell, the first o2 sensor(s) determine if the engine is running rich/lean. The after-cat o2 sensors monitors catalytic converter performance.

Chances are, the o2 sensor is faulty or it got beat around when the exhaust work was done.

 

x2. the muffler install and ur o2 goin at the same time is purely coincidence.