Carrier gas furnace code 34

Old 11-05-16, 09:24 AM
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Carrier gas furnace code 34

I have an older Carrier 58rav070-12. LED code is display 34 which, per the manual, means IGNITION PROVING FAILURE. When the furnace starts, I see a flame igniting and a fan kicks on. It stays lit for about 20-30 seconds then flames turn off. The fan stays on for another 30 seconds or so then turns off. Tech came out and replaced flame sensor but issue persists. He's now saying it might need a new control board - or that it could be another issue.

Beyond the control board, what else could be the issue? Is there a definitive way to diagnose whether it is the control board (short of replacing it)?

Old 11-05-16, 10:36 AM
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Welcome to the forums.

Unfortunately the methods used don't sound like a particularly swift technician. A flame sensor is a piece of metal rod. They very rarely go bad unless physically damaged. When it gets used it builds up a silica coating which is removable with steel wool or a scothbrite pad.

Usually if a flame sensor problem the flame will go out in a few seconds after lighting. However, a technician would connect his test meter in line with the flame sensor to test its operation before replacing it.

The next step is putting a test meter (microammeter) in line with the flame sensor.

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Old 11-07-16, 12:22 PM
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PJ Max describes the procedure for troubleshooting this problem, which your repairman didn't follow if you described what he did accurately.

Obviously, you don't want the main burner gas to stay on if it fails to light off the hot surface ignitor!

To prevent that, the furnace uses a flame rectification system to verify that the main burner has lit.

An AC voltage (usually 24 VAC) is connected to the flame sensor from the circuit board by a wire.

When the main burner lights, the flame sensor is engulfed by the burner flames, and a small DC current will be rectified from the AC voltage and flow through the flames to the sheet metal ground and back to the circuit board.

The circuit board detects if that DC current is present, and leaves the main burner gas on if it is there.

If there is no DC current, the main burner gas is turned off by the circyuit board after 3-5 seconds, and the ignition sequence gives another ignition trial, which is what you are experiencing.

IF the flame sensor has become coated with a buildup of invisible oxides, the flame wont touch the flame sensor, just the oxides, and the main burner will shut off.

Cleaning away the oxides is easily done.

What you need to do is to measure the AC voltage being applied to the flame sensor when the HSI is warming up. IF you have no 24 VAC to the flame sensor, you have a bad circuit board.

If you do have that 24 VAC present, measure the DC microamps flowing along the flame sensor wire when the main burner is lit and the main burner flames engulf the flame sensor.

You should measure zero microamps when no burner flame is present, and you should observe that go to a 3-5 DC microamps when the main burner flame is on.

(1 microamp= 1/1,000,000 amp; 1 milliamp = 1/1000 amp)

If your aren't getting the correct microamps, you probably have a bad circuit board.

The repairman did an inadequate job of diagnosing the problem. You should call the repair company and have them send a person who can check out the problem properly, and they should do that at no charge in my opinion as a retired gas furnace repairman.
Old 11-07-16, 03:29 PM
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Just one additional thing to add to SP's remarks. The AC supplied to the flame rod sensor can be well over 24vac and upwards to 90vac before the flame is established so be careful testing and set your meter higher first.

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