I've found this problem posted a lot but haven't found a solution. I have a 1998 Explorer Sport, 4.0 EFI (Not SOHC). It has 106,000 miles and runs perfectly, cold and hot.

The MIL light comes on all the time. I got a codes scanner and it shows P0171 and P0174. I clear the codes and can go from a couple hours to a couple weeks before the codes flag the light on again.

I took it in for a required smog test (California) and the emissions all pass but the car fails only because all the monitors have not run. I check the monitors every time I start it up to see if they have cleared. This last time all cleared except the catalyst monitor when the light came on again. The P0171 and P0174 codes stayed pending a few days before the check engine light actually came on.

I have cleaned the engine and examined all the vacuum hoses with no luck. I checked and cleaned the MAF sensor as some suggested with no luck. I checked and cleaned the PCV valve and hose with no luck. I've sprayed carb cleaner and starting fluid around the lower intake manifold but didn't notice any change.

I'm not sure how to diagnose the O2 sensors. There is no surging at idle, cold is around 1100 RPM and hot around 800, both are steady. I don't have the cash to just replace them without being fairly certain what the problem is.

I just need to get it past the smog test. Help?