E05 yet again with Mastertemp 400

EzraAbishai

Silver Supporter
May 24, 2020
5
Elizabethtown, PA 17022
Hi all, I've read a copious amount of threads about the E05 error however thought to ask one last time before spending the $300 for the control board. I get the SFS light and the "push ON" reading just stays at 80 then comes the E05. This is after the heater ignites and seems to run just fine - warm water flowing from return line, heat from the Stack flue etc.
I've replaced Thermistor (was broken - didn't throw error funnily enough) and Thermal regulator (tested after purchase before installation) and ignitor (broke this one when trying to clean it lol).
I have also broken open the entire unit, cleaned the cone (inside and out) that is inside the "heater tube", dismantled and lubed the fan assembly. The heater coils appear to be fine, no erosion around the copper on the manifold, I did not run water through each one however there was no visible "clogging" and no indication of any leaks, the basin was dry (a little rust which I vacuumed out).
I've checked continuity on the white wires that run to the SFS and have replaced the SFS (which still reads 3.6-4.0 MOhms).

I did clear some loose plastic (haven't identified where it might have been from) out of the bypass valve which appeared to be in great shape otherwise.

Are there any steps / ideas I have not listed that might be causing the 80 SFS temp reading and the eventual E05 or am I just looking at a control board?
thanks,
Rob
 

swamprat69

Well-known member
Mar 30, 2019
246
Las Vegas, NV
This below appears to be the troubleshooting flow chart for SFS. The first step appears to be measuring the exhaust temperature. Was it 450-500 degrees within 3-5 minutes or below 250 degrees? You have replaced the thermal regulator and checked that it opens at 120 degrees. You have checked the heat exchanger for leaks, liming, or soot and see no obstructions or scale inside. The next step appears to be checking the gas supply pressure and volume? SFS should only show if the exhaust temperature is 450-500 or below 250 degrees. You can only tell this by measuring the temperature of the exhaust. You know that you have a good SFS sensor from replacing it and measuring the resistance (3.5-4.0 Mohms and your reading is in range). After reseting, getting an E05 or E06 error asks you to recheck gas supply, thermal regulator etc.. Is your exhaust temperature in the high (450-500) or the low (250 or below) range that would cause SFS fault?

Capture.PNG
 

EzraAbishai

Silver Supporter
May 24, 2020
5
Elizabethtown, PA 17022
JamesW and Swamprat69 thanks for the responses! Swamprat69 - I was following that page although a bit randomly lol and my only concern about measuring the fuel supply pressure and flow is I don't have any device to do that with, beyond that the furnace fires and runs for about 30 seconds before the E05 pops up. Would it fire and run if the fuel is limited? We have a buried 250 gal Propane tank about 10 feet from the heater and nothing has changed with any equipment that I'm aware of. I'll see if I can find our cooking thermometer to check the exhaust temp, it definitely gets as hot as I remember it getting usually, I will note that both the heater "basin" (where the coils are) and the water exiting the unit are warm.

thanks again, I'll report back here if I can measure the temp!
 

JamesW

TFP Expert
Mar 2, 2011
19,396
Once you start the heater, press and hold the On button until the exhaust temperature is displayed.

If you get 80/40, the board is not reading the SFS.

If the SFS and wiring are confirmed to be good, the problem is with the circuit board.
 

EzraAbishai

Silver Supporter
May 24, 2020
5
Elizabethtown, PA 17022
Just reporting back, thanks again JamesW. I got the panel in and heater is running beautifully, I agree with other comments that the LCD is not as good as the LED display but it's all good! Now to balance the water and get rid of my inline Chlorinator, I'm picking up on the liquid chlorine being the better option. I used to have a Chlorine generator but they're very pricey to replace and I'm not sure I want to go that route again...
 
  • Like
Reactions: JamesW