I've written about this code frustrating the heck out of me for a week or so now, the info on the www is pretty scant on the 159 code, so I figured I'd jot it down in case someone needed it in the future.

Long story short, on our project 1994 F150 we'd replaced nearly every sensor in the truck.

MAF Sensor
IAC Valve
TPS Sensor
Air Charge Temp Sensor (At air filter)
O2 Sensor
EGR Valve
EGR Solenoid

Initially, before I was involved on the project, my son replaced the MAF sensor because he was getting a 157 Code and it would not go away. I tested the MAF sensor following the instructions in this article. Which was very helpful.
https://easyautodiagnostics.com/ford...sensor-tests-1

At that point, I found a dead connection from PIN 9 on the PCM and ordered a new one. The PCM was clearly bad and needed to be replaced. And so we replaced it, considerably easier than you'd think replacing a truck computer would be. Did it fix the problem? Not exactly.

The trucks symptoms were hesitation on the throttle, struggle to idle and stay running, extreme loping idle. Running rich, smell of gas. It just ran like garbage.

Once the new computer was in place, the 157 code vanished, but we started receiving a 159 code. As I mentioned, information on the www for 159 code was scant and/or not complete. Basically, it could be anything including
IAC, EGR, Vacuum Leaks, Air getting in the system anywhere between the Air filter and the throttle body.

Upon further investigation, I realized that the truck had 30lb injectors in it. (We'd just recently purchased it). Off comes the plenum, to get cleaned and painted. Might as well clean and paint the throttle body too while I'm at it.

Once the new 19lb injectors were installed, I was convinced that the 159 Code would go away. But I was wrong.

So, the next step was to start looking for vacuum leaks. I'm 52, I don't hear great so I'm certainly going to struggle hearing a vacuum leak, so I built a smoke machine using the instructions in this video.

While I had the plenum off and was cleaning up the engine bay, I was taking note of the vacuum lines, and noticed that they were in remarkably good shape, so my confidence was low. Regardless, we rand the smoke into the vac lines and it worked great! EXCEPT there were no vacuum leaks.

At this point, we're at ZERO KOEO codes. And a single, pesky 159 KOER. Frustrating.

I had to make a run first thing this morning and decided to stop into the local parts store. They had a MAF sensor in stock, I picked it up. Mind you, the MAF sensor that's currently on the truck is less than a week old and has moved a total of about 30ft.

Also, when back probing the MAF connector, EVERYTHING was working properly, we had voltage, the voltage changed smoothly on acceleration, everything was good. Everything EXCEPT the MAF.

We swapped the week old MAF for a new one, and voila! No more code.