Mastertemp 125 heater won't fire

oregon_mark

Silver Supporter
Jul 29, 2020
7
America/Tigard, Oregon
Hello,
I have a Mastertemp 125 heater (3 yrs old) that stopped working about 3 weeks ago. It worked fine until then and now it won't fire. No diagnostic LED's. The blower comes on, I can smell gas, lots of clicking noises but it never fires (goes through 2 cycles) and then I get the 3 flashes from the ICM for ignition lock-out. I replaced the HSI, no change. I had a Tech out and he diagnosed that I had a bad gas control valve. I replaced the gas control valve - no change. For fun I replaced the stack flue sensor; again no improvement. I went through the troubleshooting manual and I have 18 VAC to the igniter, which I assume is OK since all it says is anything but 0 or 120V (120V system) is OK. Help!
 

swamprat69

Well-known member
Mar 30, 2019
321
Las Vegas, NV
18 VAC is too low for ignitor, should be 120 VAC. Where are you measuring ignitor voltage? Voltage to ignitor is initiated by the blower air pressure switch closing. I would check the air pressure switch.
 

danpik

TFP Guide
Jun 4, 2012
1,749
western NY
If you can smell gas then the safeties all are OK and the igniter should be OK. Sounds like the burner orifices may be plugged up. systems like these will not allow the gas valve to open if the igniter doesn't work or if one of the safety switches is opened (not allowing signal to pass)

Dan
 

swamprat69

Well-known member
Mar 30, 2019
321
Las Vegas, NV
If the ignitor is not heating up due to inadequate supply voltage it would result in the symptoms described. If the airflow switch is closing, all connections at the ignition control are clean and the ignition control is getting the correct power and 24V inputs, then I would suspect that the problem might be with the ignition control ignitor relay.
 

oregon_mark

Silver Supporter
Jul 29, 2020
7
America/Tigard, Oregon
18 VAC is too low for ignitor, should be 120 VAC. Where are you measuring ignitor voltage? Voltage to ignitor is initiated by the blower air pressure switch closing. I would check the air pressure switch.
I’m measuring the voltage at the connector to the igniter. All the manual says is that it should be something between 0 VAC and 120VAC since it’s actually an intermittent pulse.
 

oregon_mark

Silver Supporter
Jul 29, 2020
7
America/Tigard, Oregon
If the ignitor is not heating up due to inadequate supply voltage it would result in the symptoms described. If the airflow switch is closing, all connections at the ignition control are clean and the ignition control is getting the correct power and 24V inputs, then I would suspect that the problem might be with the ignition control ignitor relay.
I don’t see an ignition control igniter relay?
 

JamesW

TFP Expert
Mar 2, 2011
20,430
If you have the heater wired to 120 volts supply, the voltage to the igniter should read 120 volts.

The voltage is only pulsed if the incoming power supply is 240.
 

JamesW

TFP Expert
Mar 2, 2011
20,430
If the incoming power supply is good but the voltage going to the igniter is not right, I would suspect that the ignition control module is bad.