Mastertemp 400 control board dead?

generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina
Hi. Bought our house this past July. The heater - Mastertemp 400 - worked fine. We used it at least once per month through November. At one point - I think in August - Error 01 (Thermistor) was a problem. I replaced that, but later found the real problem was the connector on the control board. Cleaned that up and it's been fine.

Tried the heater a couple of days ago. It goes through the normal display cycle 128, r=8,..., current water temp - which is 41F. Fan runs. Unit fires up. Stack puts out warm air. But after 6 seconds (every time), the heater shuts itself off. After that...
  • Mild smell of gas that quickly goes away.
  • The fan keeps running.
  • "Heating" light is on.
  • No error codes.
  • No internal error LEDs. (The "Spa mode" green LED is lit.)
The troubleshooting flow chart says cycle power and try again. So shut down the breaker for the whole pool area for a long 10 count. Tried again - actually a total of 6 times over 2 days. Same result every time.

This is not a case where the heater stops and then restarts itself (which I've read can be regulator problems). Once it shuts down, it stays down for at least 15 minutes, which is the longest I've waited before turning off.

The next thing in the trouble shooting chart with these symptoms is "replace control board." Before I plunk down $300, is there anything else I should try?

A couple of other facts:
  • I'm confident there aren't flow issues. Cleaned the filter a month ago, and pressure has barely changed. Also can see obvious churn in the spa. The jets are putting out.
  • Inside of unit looks normal. No evidence of animal nesting or weather damage. No leaks.
I really appreciate any and all help and will happily return the favor in the future if I can.
 

HermanTX

Gold Supporter
May 20, 2020
1,588
Katy TX
Pool Size
14600
Surface
Plaster
Chlorine
Liquid Chlorine
Hi. Bought our house this past July. The heater - Mastertemp 400 - worked fine.
Did the seller provide a 1 year home warranty policy? Did it include pool equipment? That is pretty common in Texas to have the seller purchase the home warranty, not sure what the routine is in NC for home purchases. If you have it, then the warranty should have you covered for repairs.
 

generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina
Did the seller provide a 1 year home warranty policy? Did it include pool equipment? That is pretty common in Texas to have the seller purchase the home warranty, not sure what the routine is in NC for home purchases. If you have it, then the warranty should have you covered for repairs.
Heh. Yes we have a home warranty, but it doesn't cover the heater. :-( Great suggestion though. Glad you made me check!
 

generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina


Also, check the gas pressure going into the heater before it tries to light and as it tries to light.


Wow. Obviously I'd missed these documents. Flame sense sounds like a great next direction to investigate. I'll give it a try. Will probably be a while before I can get to it, though.

I don't have an instrument for checking gas pressure. Will have to figure that out.

Thanks very much for the pointers!
 
Last edited:

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
24,960
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
When the heater shuts down if there are no diagnostic LEDs lit on the back of the control board then look for a flashing LED on the Fenwal box.
 

generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina
When the heater shuts down if there are no diagnostic LEDs lit on the back of the control board then look for a flashing LED on the Fenwal box.

Right! Figured that out from the references that @JamesW linked, and this is great confirmation. Super-helpful. Really appreciate the lead.

Amazing that the Pentair docs completely skip Fenwal error codes. I'm guessing I'll find it's blinking "lockout".
 

generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina
Was finally able to get back to this today. As expected after all the comments (thanks again!), the diagnostic light in the Fenwall box was flashing 3's, which is "lockout." So it looks like flame sensing is the problem.

I was able to clip a voltmeter on the flame sense FC +- pins. When the burner fired, I saw a wobbly 0.6 volts, which is not enough to signal there's a flame, so gas valve closing makes sense (pun).

I retried just to verify. This time the voltage got up to 0.8v, and the burner kept going for 5 minutes, then died.

Tried once more just for fun, and we were off to the races. After the burner had been running a few minutes, the FC pins showed 2.55v, which is well into the correct range. It kept working. I heated the spa water from 41F to 98F and enjoyed a couple of hours of bliss while the heater cycled about 10 times to maintain temp. Everything working perfectly.

Pic of the action:
PXL_20210109_203854207.jpg

Please don't laugh at my Harbor Freight meter... It works ;)

What could explain this? I'm thinking ...
  • something got wet that shouldn't and the burner finally dried it out?
  • me fiddling with the FC pins jiggled a faulty connection somewhere?
  • the igniter ground is flaky; I should install the extra ground wire?
  • the heater gods are crazy?
Tomorrow I will try this:
  • Install the extra ground wire as suggested in the Flame Sensing article.
  • Use contact cleaner on all the push-on connectors on the Fenall board. Also tighten the screw connections.
Those are not real fixes. Just guesswork. What do you all think is going on here?

Again, many thanks.
 
Last edited:

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
24,960
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Check all the grounds for any signs of corrosion and clean everything up very well.

The return path for the flame sensing current flows through the grounds. If not a perfect electrical connnection the low current will get lost.
 

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generessler

Active member
Dec 13, 2020
27
Chapel Hill, North Carolina
The meter is not good enough for this test.
You need a good true RMS meter to get a valid reading on this test.

Well, that's as may be. But the manufacturer's data sheet says this:
Alternately, a Digital Voltmeter may be used to measure DC
voltage between FC+ and FC- terminals. Each micro-amp of
flame current produces 1.0 VDC. For example, 2.6 VDC equates
to 2.6 μA.
 

JamesW

TFP Expert
Mar 2, 2011
24,149
If you're going to be doing electrical work, you should get a decent meter.

A meter that you get for 99 cents at the discount store is not a good quality meter.
 
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