Is everything ok on the combustion end? You have lean exhaust, which doesnt necessarily mean you aren't giving it enough fuel.

Thanks for the resonses. I have the '98 manual and it also states the pressure should be 41 to 46 psi. I checked the fuel gauge on my tires and it matches with my other tire pressure gauge readings. I noticed when I ordered the pressure regulator that Isuzu changed the regulator in March of 1998 to the one used today. My Trooper was built in Oct of 1997 and used the old regulator. Anyone have a manual for a 1997 Trooper to see what the spec is for fuel pressure psi?

Your suggestion on changing gas is worth a shot. I leave in Colorado and we use Ethonol in the gas during Winter. However this problem started before November but it's still a possibility.

I plan on having the fuel injector system cleaned. The error codes are reading on both banks (sides) of the exhaust (i.e. both rear O2 sensors). My guess is that if it were a fuel injector it would only read on one side although it is possible more than one injector could be partially plugged.

Again thanks for the input.

I replaced the MAF today and not more CEL errors. Somehow the ECU must have been getting false readings from the MAF which resulted in not enough fuel being sent through the injectors. Thus the CEL codes for lean conditions.