I am having a problem with a check engine light fault code p1151, lack of upstream heated o2 sensor switch, sensor idicates lean, bank 2. At first I thought it was a bad o2 sensor and replaced it out. Now I am finding that not the case. The sensor seems to be getting little signal(from my airfuel gauge which could indicate lean or something else wrong). The truck idles rough but runs ok once I get going. What could be wrong?

 

You may possibly have a vacuum leak. Check all your hoses, especially the PCV valve hose. I'm not sure on an L, but the 5.4 in the Navigator/Expedition are nortorious for an elbow running from the manifold to the PCV valve deteriorating and causing a vacuum leak. Good luck and hopefully you will find the problem.

 

Could very well be a vacuum leak--on a '97 truck but how many miles on the fuel filter? If over 5k, could very likely be clogged and you will have driveability/detonation problems. Change it now of the miles are on it.

Dan