Hey guys, I'm trying to diagnose a problem but I'm not getting any clear cut signs (at least in my weekend mechanic experience). Any help is appreciated.

I have a 2000 HX w/ ~150K. Only mods are an intake and suspension. I've been doing regular maintenance on it (including plugs, plug wires, fuel filter, air filter, rotor & cap). My gas mileage used to be pretty good, but within the past month or two it's dropped about 8 mpg. The only code I've thrown is for the Primary O2 sensor, which I replaced along with the sensor after the cat. The car has been running fine (no shaking, no low/weird idle) in spite of my loss of fuel efficiency.

Two days ago my CEL came on and I got it scanned and it was for the Primary O2 sensor. Car was still running fine so I had the light cleared to see if it would come back. It had stayed off so far but today as I was driving back from the store my CEL started to flash. The car was still running fine so I just hurried to get back home (only half a minute away). No codes were thrown and it starts up and runs/idles fine still.

I've been trying to research the loss in my mpg, and it seems like it could be a possible cat failure. But I'm not getting any clear symptoms of one (at least what I've read online - Smells like eggs, cat rattles, sluggish [Hard to determine in my car h:], etc...).

So if anyone could throw out a good suggestion of what I could do/check, I'd much appreciate it.