[Stephens]

I just got this DTC as well. I am running Borla UEL and a muffler delete. No tune.

First off, this DTC is neither a lean or rich code.

From what I have found on other sites and in relation to other cars, this code is caused by a few things.

Ultimately from the upstream and downstream sensors not seeing eye to eye. On stock tune, the ecu has a parameter range set that the ratio of sensor 1 and sensor 2 must fall in. Removing the cat can make the ratio fall out of this range.

I also found a few other possible causes:

- Intake Leak(s)
- MAF/AFM
- Vacuum Leak(s)
- Exhaust Leak(s)
- Bad o2 Sensor (Secondary or Primary)
- Loose o2 Sensor (Secondary or Primary)
- Leaking Injector(s)

The most common thing noted on other forums is exhaust leaks and loose o2 sensors. Though, some of those guys had no leaks and the only fix was an adjustment in their tune or a tune if they were running stock maps.

Apparently, it's a common problem on the newer generation Mustangs when any piece of the exhaust is changed.

It's also been noted that the code occurs more frequently in colder weather and that the ecu checks for this "ratio" in closed loop scenarios.

As for me, I have had no drivability issues and p219a has only shown up once. I do get p0420 often because of no cat. I am going to check for exhaust leaks and go from there, but I am not too concerned with the code at the moment.