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Secondary use will be on control board in blower compartment.See links below.Remove the fuse and see if it is blown.
Absolutely.If it pops again you have a short and that will requires some troubleshooting.
The 21 error is stuck gas valve relay or miswire.
Check to see if you are getting 24v to valve with furnace off?
Yes I saw and changed my response (typo)
You have voltage to gas valve with thermostat OFF?
With thermostat OFF we should get 0 voltage at gas valve.On al for heat when burners should light we should get 24vac
At gas valve
The error is saying the gas valve is getting 24vac when it should not or the relay built into valve is stuck
OK but still have the 21 error?
You could possibly not be getting sufficient pressure to valve, Can you check or have him check inlet pressure at the valve?
I understand.It appears you have gone as far as you can without the tester to check pressure. Next step is to get the pressure checked or replace valve and see if that solves.I just hate being a parts changer.
If you have further questions, please let me know.
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