CEL came on a couple times on my 92 f250 460, but would not stay on, only intermittant every once in a while. I scanned the codes and got 41 on koeo and koer. cleared it and put in a new O2 sensor as it probably needed one anyways. Drove it another 60-70 miles and on and off several times. CEL light has not come on but I still get the code 41 on the koer test. Its the only code I am getting, seems to run fine as well. What can I check next?

 

If you're sure you got the code cleared then I would monitor the O2 sensor signal during operation. Someone else might have a better (or easier) idea, but I've done this before & it helped me figure out what was going on. Consult a wiring diagram to find the signal & signal return wires for the O2 sensor then hook a voltmeter to the signal & signal return wires (be sure not to accidentally check the resistance of this circuit as that could damage the ECM). Use some extension wire so you can run it into the cab so you can see the meter while the truck is running. After it warms up you'll see one of three things:
1) very low voltage, may less than 0.05V. This would indicate the O2 sensor isn't reading. A bad ground can cause this.
2) low voltage, around 0.2V. This would mean that the truck is actually running lean, at which point you could start looking for a vacuum leak or something.
3) fluctuating voltage, anywhere from 0.2V to 0.9V. This is what the voltage should do, but if it spends most of the time around 0.2V then its basically the same problem as #2 above.

Before you go to that trouble, though, you might want to try resetting the KAM again (battery disconnected for at least 10 minutes)

 

OK so before I get into the O2 testing, I checked general engine vacuum and it seems fine, 17, but thats what all my other cars run at this 5000 altitude.

I tested fuel pressure and to me it seems a bit low. 29-30 psi at idle and elevated rpm. doesn't drop when I crack the throttle. 35 psi engine off.
and 35psi with vacuum disconnected at the regulator. To me it just seems low overall, but the haynes manual states 30-45 psi at idle.
Thoughts? I don't want to throw a $50 regulator at it and have it not change anything overall. I think the pump is fine as it would show up in the testing as low or irregular, right?

 

Bump

I also have put about 200 miles on the truck since the O2 and changed the fuel filter, gained about 1psi. No CEL and the truck runs real good getting 14mpg mixed bag.

Should I really worry about it if the truck runs all right and its not throwing a CEL or in the KOEO test memory?

I still get the code in the KOER test though, why does it not show up in the KOEO or CEL?