Where is the fan limit located on the Carrier 58CVA110-22 furnace? This furnace is flashing code 34.

A Carrier 3-4 diagnostic code inicates that the furnace burners aren't lighting or that if lighting the fact that they light is not being detected.

The most common cause for that problem is a flame sensor coated with invisible oxides that needs to be cleaned with a wire brush or fine sandpaper.

The flame sensor is on the side of the furnace opposite the hot surface ignitor, and is a rod about 1/8" in diameter sticking up into the burner flames with a wire coming out the bottom that goes to the circuit board.

Remove the flame sensor, clean it and try again.

Based on the diagnostic code, I don't see a reason to suspect a limit switch problem without additional information.

I cleaned the flame sensor and it still had the same symptom. I even changed the flame sensor with an identical sensor from an operating furnace and it still shut down and flashed the 34 code.

Check to verify that you have a good ground between the ignition control/circuit board and ground.

Use a multimeter to measure the AC voltage at the flame sensor and the furnace C terminal or chassis ground while the thermostat is calling for heat and the inducer motor is running.

If the voltage at the flame sensor is zero, check the wire carefully to be sure it's intact going back to the circuit board.

The circuit board uses "flame rectification" to verify that the gas burners have lit. This phenomena of electrical theory applies an AC voltage to the flame sensor, somewhere between 24VAC and 120 VAC depending on the circuit board design.

When the flame sensor is engulfed in flames from the burner, a small amount of the AC voltage is converted to DC current and flows through the burner flames to ground and back to the circuit board. If the circuit board detects that DC current, the flames are proved and the burners stay lit.

Having the circuit board grounded properly is thus critical to the operation of the furnace.

If the circuit board is not applying the proper AC voltage to the flame sensor, the circuit board is probably defective.

Are the burners turning on and then shutting off after 3-5 seconds or so? This is the typical operation of a furnace that is not verifying that the burners are lit.

Each time the burners light, they stay lit, the blower comes on and about 20 or 30 seconds later the burners shut down. Sometimes there is an intermittent "click click click" sound and the burners shut down.Also sometimes after the burners light it will "click" and make the flames jump. I put a new control board in and it made no difference. My brother-in-law thinks I need to replace the wire from the flame sensor to the board or connect the wires from the HI lug on the gas valve to the M lug to see if there is a problem with the HI function.

SP. Very nice description of flame rectification, well done.

-unplug the wire from the flame sensor.
-get your multimeter
-set meter to dc mil volts
-take one test lead and plug into the disconnected wire
-touch the other lead to the flame sensor terminal
-start the furnace
When the flames ignite the meter will display dc millivolts
If the volts are under 1.6 I would replace it. Or clean it and see if you can get it up between 2.0-4.0 volts

Hello Headscratcher,

Since the Carrier 3-4 diagnostic code is indicating that the burner is being shut off because of flame failure, wiring to the gas valve is unlikely to be an issue.

The clicking noise you hear is probably the circuit board detecting a flame failure repeatedly, but then detecting the flame again before the gas valve shuts off. Do you notice ANYTHING about the flame or furnace that is associated with that clicking noise?

Observe the flame sensor carefully. Are the flames being blown away from the flame sensor at any time, which would cause a flame failure?

I would use a wire brush to clean the burners in the area around the flame sensor, in case the burners are coated with oxides the way the flame sensor can be coated, preventing current from flowing to chassis ground.

The wire from the flame sensor to circuit board could be a problem. Replacing it would be fine, or polish the connections with a wire brush and check the wire to be sure you are getting good electrical connections at the flame sensor, along the wire and at the circuyit board. In particular, does moving the wire around cause the clicking noise you describe?

And check the ground connection and ground wire from the circuit board to chassis ground carefully to be SURE that the connection there is good. If the current fails to get from chassis ground to the circuit board, you will get flame rectification failure, and a poor electrical connection is a good cause of the erratic clicking you have noticed.


Fun, eh?

Oh boy, you said it, fun indeed.:)

Just to add to the list of items to check. Gas pressures would be a good next step after you have done all test and maintenance stated above. Extremely low manifold gas pressure can cause low flame signal also. One step at a time with this type of problem. Please keep us posted.