Just bought an '06 GT w/ 4.6. Filled up with Exxon 87 octane unleaded and noticed decrease in power and engine light came on. OBC-II code said catalytic system was operating below optimum. Manual said to use premium 87 octane. Any suggestions?

 

I seriously doubt the problem is the gas. Its only a coincidence. Your using the suggested octane for your motor, so its fine.

As for the loss in power and the CEL, those two are related. What was the exact code?

 

Was the car purchased new or used, does it have an aftermarket CAI, do you know if it's had a custom tune put in it? which might require higher octane.

 

bought the car used from a dealer so I don't know anything about it's tuning or anything else. The exact code was P0430.

 

one of your cats or 02 sensors may have failed

 

Here, let me google that for you.

http://www.obd-codes.com/p0430

Most likely either an exhaust leak or the pre or post cat O2 sensor is bad. Its rare for the cat to go "bad" compared to the other causes, but it does happen.

 

Exhaust system looks to be in great condition end to end so I don't think I have a hole. Will get my nephew to hook it up to the puter and see about the timing because that's now my prime suspect since this started as soon as I filled the tank. Thanks to all of you for your input.

 

The whole can be the size of a pin. Could be a bad weld, or a loose connection or leaking exhaust manifold gasket or cracked exhaust manifold etc etc. A visual inspection of the exhaust pipes is virtually useless. Like finding a needle in a hay stack. Best way is to start the vehicle up, crawl underneath and have a listen. Easier with a mechanics stethoscope.

 

@petrock, listening is a good idea except that I'm about as deaf as a door ****. Kids ask me why I'm wearing two Bluetooth's and why do I need two phones.

 

Heh, get your kid to do the listening... Why else did you birth them? :-)