One upstream and one downstream of each cat. I think the downstream ones tell the pcm the condition of the cats.

 

O2 sensor would be my guess. If it was a fuel regulator or fuel filter problem the computer should compensate somewhat at part throttle and mileage should be pretty close. The O2 sensor is what tells the computer how much fuel is needed so if it goes bad and sends the wrong signals the computer doesn't know that and just adds fuel accordingly.

The front O2 sensors are the only ones that need to be checked.

 

Has he changed tires since the mileage drop?? Increased them in size? I know a heaver tire will kill the mileage.

 

Mice nest in the airbox?

Honestly, that's a problem with my truck and my girlfriends car. We got three cats too. The bastid's move in even if the vehicles are run every 24 hours.

 

He has, but they're the same size. But I guess ya never know. You would think Highway and Town milage would drop some. 23/17 before, you'd think it would be 21/15 or something like that. Not just 17 all the time. I'm assuming if you scan the PCM you'll be able to tell which O2 Sensor is shot? Sending the wrong voltage and such.

And two cats......sweet jesus.