I've been working on a tac 20 that won't take hall calls. It runs fine on inspect, rest power and it will run a week and quits agine. While it was down today i viewed the faults and there was none. Any idea where to start with this?

Got IMS?
If so, type in PCL (permission change log) and see what or if any changes ocurred besides automatic.
No IMS? The only thing that would make that worse is if the job is LON.
No faults, but you have to reset it...odd. Have you tried Fire Phase 1 while it's apparently not working? I ask this because I've seen more than a few times flakey key switches that while in the "off" position, it seems to block hall call inputs, but car calls are OK. You don't mention, but will it take car calls when it won't take hall calls? HPI registering floor number? Fault relay picked? Starter has LCD readout ("Motor Stopped")? Check for proper clean ground terminations, check controller terminal CONNectors for proper plug seating and wire integrity into the plugs. And if it is a LON job, find the LN card and see if (when it's not responding in a week or so) the flash rate of the LED. Should be like once every 1.5 seconds or so. If it has a rapid flash rate, you'll have to find the bad L24 card. Have fun with that. These are some good places to start.

Did you re-learn the shaftway?

Thanks for the help guys going back to that job Friday will let you know what I find.