@qqzj reading through this figured I'd share a recent diagnose and repair that i recently had on my F150 hopefully will give you a sense of what you might be up against if you try to do this professionally. First off I'm not a professional mechanic but would consider myself and advanced DYIer and have tackled some fairly challenging repairs on my own vehicles over past several years. Recently noticed I was having trouble filling up my fuel tank would only go about 1/2 tank before gas would come out the neck as I was filling it. No codes or drivability issues just couldn't fill the tank without going exceptionally slow at the pump. A few weeks later I got a code and check engine light for something like unable to draw down fuel tank vacuum (it may have been P1450 IIRC but not positive about that). Asked alot of questions and watched a lot of you tube videos and received many various suggestions on what could be causing problem. Purge valve, vent valve, damaged fuel neck, missing fuel neck flapper (my truck doesn't have one but at one point I was trying to see if it somehow became unattached and was partially blocking tank neck), bad gas cap, clogged EVAP canister were all potential possibilities. Rather than just throwing parts at problem I tried my best to diagnose problem. Checked the purge valve and found it to be working properly and put vacuum Guage on it and actuated solenoid through scan tool and confirmed it was working, disassembled the EVAP canister and confirmed it was not clogged (major PITA project), tried my best to confirm nothing was stuck in filler neck, ran grease around fuel cap, reset codes and tried to see of the code came back. Finally had to drop the gas tank and test fuel tank pressure sensor and found it to be bad. The pressure sensor is part of the vent hose assembly that runs from engine bay to tank and back to evap canister. Major project to replace. To top it all off none of the auto supply houses show this part as part of the system I had to go directly to Ford and it was a $300+ assembly. The new part number used a different connector so had to buy a new harness and figure out wiring. At end of the day I tracked down problem and repaired it myself. Had I listened to all the tips it would have potentially cost $1000s to throw parts at it. I'm sure someone with the right subscriptions would have diagnosed issue more quickly but that costs a lot of money in addition to test equipment. Just think about what you might be up against before jumping in. I'm sure this is fairly representative of typical problems you might face on a daily basis if you were doing this professionally. Good luck!
). I wouldn't have paid shop-money to avoid all that, but...