1) GE timers don't have override switch, but if you raise all the timer segments, the circuit to water heater will be completed.
2) Open link below to identify timer and see wiring diagram, and wiring instructions.
http://waterheatertimer.org/How-to-wire-GE-15207-timer.html3) Override timer using the wires:
Take a photo of wiring so you can re-wire correctly on new timer.
There is 1 black and 1 white Line wire coming from the breaker.
And 1 black and 1 white Load wire going to water heater.
Connect black Line and Load wires together on same terminal.
Connect white Line and Load wires together on same terminal.
This will complete circuit to water heater and by-pass the timer.
4) Buy new timer at Home Depot.
Reconnect wires on new timer.
Return defective timer in new box.
Keep new timer clean, and swap out whenever needed and give yourself extended warranty.
5) This is the first report of GE timer going bad that I've seen on Fixya.
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Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.