Every season my Jandy Lite 2 millivolt heater is a bear to start. Sometimes I find something wrong like a bad pressure switch or a low output generator. But even after replacing those parts, and with everything, measuring fine through the whole safety and control loop, on the first startup of the year I never can get the main burner to fire without waiting long periods of time.
Some years I do nothing other than leave it switched on, and after an hour or so, suddenly it fires and is fine for the rest of the season, restarting many many times without delay.
Today was the first day I went to fire the heater this year. Lit the pilot. Flipped the temperature control switch to ON and set the potentiometer to max value. Waited. Nothing.
Went through the entire Jandy troubleshooting procedure and found 244 millivolts (control switch on) at all points along the safety and control circuit (between PP and every test point). With switch off, generator measures 511 millivolts between PP and PP/TH. Those values meet the 220 and 500 minimums from the manual.
I jumpered between the PP and TH terminals to try and bypass everything. Still won't fire. Is this all pointing towards a bad gas valve? Tapping on it with a small hammer doesn't seem to help.