Check Engine Light

I have a 98 Expo and the check engine light has been coming on recently. The code in the computer is P0171 and P0174. Both translate to Banks To Lean. I looked this up in my Chilton book and it literaly means that the fuel is too lean going through the engine. It also says that the computer in the truck is supposed to register this info and compensate for it. I reset the code to turn the light off and about 3 days later it comes on again. Any idea what could be causing this?

I was thinking maybe the O2 sensor might be going bad. Any other thoughts would be great.

Thanks...

I had a similiar problem. It turned out to be a vacuum hose was cracked at the rear of the engine. Spray some carb cleaner along the vacuum hoses, when the engine speed picks up, you have located the leak.

I had the same problem. I am one of the few people who use there expo as recreational 4x4. I thought i might have damaged something going thru deep standing water. i replaced the O2 sensor on the drivers side above the catalictic converter and that fixed the problem. i used a scanner to reset the code. it happened once more but I simply disconnected the same sensor and plugged it right back in and reset the code. no problems since. I know that this is not a quaranteed solution but it only cost $60. give it a try. you might be able to just disconnect the sensor and clean it off then plug it back in. also if it is the sensor, depending if you can stand to look at the check engine light, which I cant, you can continue to drive the vehicle without replacing a bad sensor. it will have no mechanical problems develope from just a bad sensor. but be sure you only have a bad sensor before you completely disregard it.