MASTERTEMP 400 Combination Gas Control Valve Troubleshooting


Bronze Supporter
May 13, 2017
Saint Petersburg, FL
I have a Pentair MASTERTEMP 400 natural gas heater, manufactured 8/15/2016, model# 460736 (standard model, not the HD/ASME). I just replaced the thermal regulator (original was corroded shut). I've been troubleshooting, the heater currently has the following issues:

1) Automatic Gas Shutoff Switch (AGS) is bad (measures open circuit when cold). I temporarily bypassed this bad switch for further testing.

2) Control Board is bad (I went through the "Heater Will Not Fire - C" troubleshooting chart in the manual, and determined that there is no 24 VAC between the GAS terminals on the Control Board (about 24 seconds after call for heat). I get 4.9 to 5.3 VAC open-circuit (when not plugged into gas valve TR/TH terminals, measured at Control Board connector), when plugged in (loaded) I get 0 VAC. So the 24 VAC drive circuit for the GAS terminals on the Control Board must be shot.

3) The Combination Gas Control Valve is not opening to flow gas when manually actuated (heater does not light, no gas smell, gas meter does not move). The valve is normally actuated (Control Board) by ~ 24VAC. I manually applied 26VAC using a sprinkler timer wall transformer, to mimic the timing/duration of the voltage from the Control Board. I disconnected the pink/orange wires from the TR/TH terminal block terminals (see photo) and applied the 26VAC there, so I know the wiring and ON/OFF switch is good because I can hear the gas valve actuator clicking on/off.

Question: Is there any other troubleshooting I can do on the Combination Gas Control Valve? As stated, the (electromagnetic?) actuator seems to be working (a very audible click on/off with applied 26VAC). Could there still be something wrong with the actuator? What else could cause the gas valve not to flow? Is there a screen in the valve that could be clogged? Can these Combination Gas Control Valves be rebuilt? Or if bad would I need to buy a whole new unit?

Please see second pic. The gas ball valve (yellow handle) was open for these tests. I removed the bronze cap (for future gas grill hookup) and confirmed there is plenty of gas available/flowing and saw the gas meter turning when line open.

I hate to have to replace this heater already since it's only 4 years old! If the gas valve worked, I was willing to spend the money on a new Control Board ($$$) and AGS switch ($) to get it back up and running. But if the gas valve is bad also ($$), I think it's time to look into a new replacement heater!

Any suggestions on my questions above would be much appreciated! Thanks!


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Pool Clown

In The Industry
Sep 5, 2008
Silicon Valley, CA
I would say board or Fenwal. Both high dollar components (of course). Both have a part in giving the valve voltage. The control board Tells the Fenwal to give voltage, and the Fenwal does the heavy lifting via a relay. You need to find out if the control board is brain dead, or if the Fenwal is just on strike.
Confirm (when there's a call for heat, past pre-purge) that you have the right voltage at the VAL pin on the board, and then connect and test at the VAL WIRE at the Fenwal i think its yellow with a red stripe. Testing the wire rules out mice/damage. Then just follow the voltage through the Fenwal to the valve.

Having said that.,..

If you are getting voltage, but not full voltage makes me lean towards board or maybe AGS switch that is causing some resistance. You mentioned by-passing the AGS, so, not sure there. The voltage that operates the valve must be proven through the AGS. See attachment for yourself.
Pages 30,31.
Perhaps confirm voltage on connected wire to AGS and follow backward?


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Well-known member
Mar 30, 2019
Las Vegas, NV
This seems to be the manual for your heater 472592_revG_eng_6-26-12 ( . The flow charts for troubleshooting a "no heat" situation can be somewhat confusing. The Operating Control, Membrane Pad (User interface) and Ignition Control Module (all control operations are on the 24V side of the system) all interface with each other. Understanding the "Sequence of Operation" is the key to troubleshooting the problem. The Membrane Pad will tell the Operating Control that you are asking for the heater to start. The 24 V side of the transformer has one of its wires grounded and the other wire is the one that travels through the controls for the operating sequence. The easiest way to troubleshoot is to attach one meter lead to the 24V ground in the system and trace the other 24V leg through the controls with the other meter lead. If the membrane pad is asking the system to heat the operating control should be putting out 24V measured to ground through the operating control "TH" to the Ignition Module "TH". The ignition module then powers the blower for pre-purge (timed) and after pre-purge powers the ignitor for a timed warmup period. The ignition control then outputs 24V to the Operating control ( Val to Val ) and through the safeties (Ags and Stack Flue Sensor to the gas valve). You are questioning the operation of the gas valve when you already know that you are not getting 24V to the gas valve. You need to look at the wiring diagram/schematic ( pages 31 & 32 ) and find exactly where you are losing the 24V. Have you confirmed power to blower? Have you confirmed power to ignitor? Have you confirmed ignitor is heating? Have you confirmed 24V out on ignition module "VAL" terminal after blower pre-purge and ignitor warmup ( both timed )? If you know the sequence of operation you should be able to trace the ungrounded 24V leg through the system to find where you are losing it. Will be happy to respond to any questions that you may have.

Pool Clown

In The Industry
Sep 5, 2008
Silicon Valley, CA
I suppose i should have mentioned that the manual i referenced is the Sta-Rite version of the same heater, the Mastertemp.
Wiring, components, and schematic would be the same.
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