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Art, it’s NOT always the thermopile. Some visual inspecting and use of some test equipment is in order here.
What’s the general condition of the hi-limit’s bulb & capillary routed up there inside the fryvat? If it looks banged about and abused, replace it. It’s been compromised and may have leaked a little of the gas in it that makes it work. I must point out that a hi-limit is designed to FAIL SAFE. In other words, if its functional element (the gas charge) escapes, then it fails OPEN. A v-e-r-y small leak is a hint of that feature, by it tripping at lower temperatures than it’s designed to.
Is the hi-limit bulb mounted properly? That’s VERY important. If it’s not and has nestle its way directly up against a burner tube, then it’s merely doing its job – SENSING an overheat condition.
Otherwise…
With regards the 200-250ish degree temps you stated, what tests have you performed? Is the temperature you said it heats to –
the SET POINT of the operating thermostat? or the ACTUAL temperature?You need be monitoring the ACTUAL temperature with a quality pyrometer/digital thermometer and, when the pilot goes out prematurely, check that hi-limit with a voltmeter.
If the hi-limit tests as open at a temp well below its designed trip point (450°F), then the hi-limit needs to be replaced. If that’s the case – and the sensing bulb/capillary doesn’t appear compromised, then I suggest replacing the operating thermostat too. The operating thermostat is a mechanical device and wears out over time. It might be sticking occasionally and causing an overheat condition – thereby giving the hi-limit (also a mechanical device) a work out to the point that its gotten weak and has failed.
I also advise that you check your electrical connections. ANY poor connection in a 750mV circuit (what the thermopile generates) is a weak point in the circuit. 750mV voltage isn’t enough potential to punch its way through a bad connection.
If you’ve done these things (connections are good, bulb/capillary is good and hi-limit not testing as open, other strategic voltage test points show as good), THEN you can suspect the gas valve as the culprit.