1996 F-250 Codes-What Do They Mean?

I finally got around to borrowing my friend's scanner tool this weekend and hooked up to my '96 F-250 to see why the check engine light comes on every now & then. I've also experienced an intermittant power loss so I knew I'd pull something on the codes. Here's what it gave me.... KOEO Code 111 System Pass Code No Faults Detected During KOEO Test

Memory Code 172 H02S (sensor) System Lean Bank 1

Memory Code 334 EVP Sensor Closed Valve Above Max Voltage

Memory Code 542 Fuel Pump Secondary Circuit Open PCM Fuel Pump Gnd

OK, that was easy to use the scanner & pull the codes. Now-what do they interpret to? Is the 1st code (172) the oxy sensor? I've suspected this might be bad for awhile, but don't want to replace unless needed (there are 2 of them on this truck) I don't even know what an EVP sensor is...any thoughts? What would it make the truck do if defective? As far as code 542 goes, I replaced the front tank fuel pump and filter this past summer, didn't really need it but it was getting noisy so I did it before it conked out on me. Pretty straitforward install, so maybe this code is for the fuel pump in the rear tank? Here are the specs on the truck....

1996 Ford F-250 4x4 Auto 5.8 L engine 212,000 miles, mostly trouble-free Idles perfectly smooth,never a miss When ascending a hill, *sometimes* it gives trouble-feels like you're towing a large trailer or something like that....this does not happen every time, though. It does not seem to be affected by the weather (New England, so we get everything from brutal cold to blazing hot!) Every other fill-up, I add either dry-gas or STP fuel conditioner. Mileage *used* to be a very steady 13-15 mpg (good for the size of this truck), but is currently around 7-10 Sorry for the lenghthy post...maybe someone can share some opinions or ideas? Thanks! ~The Doctor

Many times, an always lean condition can be traced to a vacuum leak. Since you are showing a higher than expected voltage on the EVP sensor, that is more indication of a vacuum problem. I think the first thing I would do is trace and check every vacuum tube for a problem. Keep in mind that a bad EGR valve or one that is stuck open can do this but, usually results in bad idle unless it is still within the adaptive limits of the ECM. Check for vacuum at EGR as engine speed is raised accompanied by voltage change. You will need both a vacuum guage and a digital volt meter for this. You might want to disconnet the battery for 10-20 minutes to clear the operating parameters for the engine. If a sensor or something is nearing limits, you may be able to see bad idle or other symptoms for which the ECM has not yet adapted.

Lugnut