my 98 ranger 4.0 check engine light came on and the codes were a bad iac valve and it also said it was running lean, so i replced the iac and the light went off. so now about 500 miles later the light came back on so i took it to advance and the code reader said it is running lean but there were no other codes and the guy went though every sensor and all look like there reading properly. What could be the problem?

 

Vaccuum leak. Most likely cause of the problem. It could be an intake manifold gasket they are popular for that problem.

 

Post All of the First & Last scan trouble code Numbers, as they can offer up good trouble shooting clues.
Did these problems come about suddenly, after some event???