Not sure if this is of any help, but I had a phantom CEL for a bit. The car had issues starting and would throw a CEL, or sometimes at idle, the revs would drop and I would get a CEL. Gave the car a tap on the accelerator and the car would be fine again. But the CEL would remain on.
It would remain on for the 5? starts and then go away by itself.
Now, I would never attempt to check the code straight away. So if it was the idle issue say, I would be going to Bunnings for instance and it would happen on the way there. I would then park the car and get what I wanted & then start the car again and go home. Pull out the
ECU Datascan & No codes found, or whatever it says, came up. This is still with the CEL on the dash lit up. I even took it to Subaru and they could not get a code.
What I found was by turning the car off, even once, within the 5 starts, before the code resets itself (CEL turns off), the code could not be read. I managed to get the code by not turning the car off, from the first instance when it came up. We managed to get the code, using the Evolution R code pulling machine, when the car was on the dyno at there. But then using my
ECU data scan I could do it by myself, again, by having the
ECU Datascan in the car with me and plugging it in as soon as the CEL came up.
You seem to know when it is going to happen, i.e:- going into 5th so maybe you could force it to happen with a borrowed, or someone with one in the car with the scanner, to see if the same thing applies that has happened to me.
It might not, but its worth a try. Especially as you seem to know when its going to happen & you can make it happen easily?
Hope this helps.