Help with SES light, code P1158

99 740iL, 49k miles. Got Service Engine Soon light, code P1158. Changed Oil Separatoin Valve, tightened up two loose intake bolts, reset light, got another service light after 80 miles. Light seems to come on at idle. Code P1158 for "Fuel Trim additve Bank 1 Low". Does that mean the voltage is low, indicating high oxygen, or does it mean that the mixture is rich? Also, seems confined to bank 1. Dealer is no help ("We can make an appointment for you next week"), local mechanic said he would think O2 sensor. Any help with this code and where to go next?

I have a 2001 740iL with 72000 miles about 4 months ago the SES light came on. I went by the local parts store and had the codes read P034 (oxygen sensor position 1 bank 1, P1158 manufacture specific code). I bought my own code scanner and replaced both upstream sensors and cleared all codes.
P034 gone, P1158 Fuel Trim additve Bank 1 Low returned after 30 mile drive.

Expert answer was replace the downstream sensors as well. I did this and again the P1158 code returned after 30 miles. I have not replaced the oil seperation valve yet but the car is not exhibiting the problems associated with that part.

I think this problem may be related voltage signal issues within the system.

I tightened loose intake bolts, cleaned the MAF with CFC brake cleaner (the real stuff in the red can that's nasty, not the environmental stuff in the green can that leaves a residue), and cleaned and made sure all hose connections on intake were tight. P1158 cleared and has not come back in 8k miles. Plenty of other stuff with this maintenance pig, but not that (knock wood). I think it was the intake bolts and that tightening them back up it finally resealed. They're easy to get to with a small (1/4" drive) 10mm deep socket and extension; five on each side. Never did change the 02 sensors, they weren't the problem.