SPN 4334 FMI 7 EPA10 DD15 Engines

I want to share my story about repair of this code. The driver called my a week ago and told me that on uphills this code pops up. Only on uphills and only this code. This code says Low DEF Pressure. I did all procedures as mentioned in PSL for this code troubleshooting(checking the screens, puling def lines and checking them, running air pressure test by DL,...). Nothing was found. I repeated the procedures three times. I cleared the code and sent truck to work. I don't like to throw parts on truck to see what will happen. The driver called again same day and said the code pops up again. What is interesting about this code that if DEF pressure goes back to normal you can cycle ignition key and amber check engine light will be off. When code is active engine does 25% power derate but when light is off power goes back to normal. Again did all procedures and checked all wirings. Again nothing suspicion was found. I was thinking ACM is malfunctioning and I was about to call the dealer. Then changed my mind and said myself lets do the dealer way. Lets change parts to see what will happen First I installed my tested used PLU and sent the truck to work . He called my back and said the light came on. Next one was replacing DEF pressure sensor and again the code came up. So now I was sure that low DEF pressure is related to DEF pump. That DEF pump I had installed myself in last August. Took out the pump's air switch valve. All O-rings were like new and it was looking like a new part. Replaced it with used tested one. Sent the truck to work. The code did not popped up again. It was gone... So the troublemaker was small pos air switch valve that is located in air port of DEF pump... $50 piece...