Carrier code 33: repeatedly, intermittent, over and over

Old 01-27-21, 04:04 PM
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Carrier code 33: repeatedly, intermittent, over and over

I've had this Carrier Weathermaker 9200 furnace for 15 years and wish to share my horror story and SOLUTION so I may help others out there. I have not seen this SOLUTION anywhere else as I have tried everything. Everything.

Pretty much this is a new furnace, the professionals (we used different ones as no one seemed to get this thing working without hiccups aka Error Code 33). They replaced the computer board, limit switch, high limit temperature sensor, heat exchangers, blower fan twice, even the thermostat, and of course filters, and the flame rod sensor. I replaced one wire I thought was the key, it wasn't. I was thinking about ordering a new wiring harness and re-wire the entire furnace, what the heck, everything else has been replaced!

I was there each time with the service techs asking questions, and trying my own things, there was no way to have confidence this furnace was going to work. Each time we left town, we hired neighbors to come over and check on the heat each day of our trips.

Does this sound familiar? So what was the problem (I am writing this with three months without a hiccup - no Error Code 33 with slow growing confidence and cautiously optimistic).

The Flame Sensor is an interesting device. When the flame hits it, it generates a tiny bit of current .000001 to .000006 amps to be exact. That light whisper of a amperage goes by wire to the computer to let it know the flame is burning and to let the gas keep running and burning. Your cell phone uses a crap load more amps.

I looked at the flame sensor as a "circuit." It has one wire to it. The metal flange of it is screwed to the flame box (the metal is ground and becomes in-a-sense, "the other wire"). It came down to "How good is the ground?"

I made the following pictures, that shouldn't be any different than yours.
1. The first ground connection is at the circuit breaker box, the conduit connector is attached to the circuit breaker box.
2. The second connection is where the conduit goes into the connector.
3. The third connection is where the conduit goes into the connector at the switch box.
4. The fourth connection is where the connector attached to the switch box.
5. The fifth connection is the connector coming out of the switch box.
6. The sixth connection is the greenfield going into that connector.
7. The seventh connection is the greenfield going into the connector on the side of the furnace.
8. The eighth connection is the connector attached to the furnace.
9. The ninth connection is the wiring box inside the furnace connected to the furnace box.
10. The tenth and eleventh connections are where the furnace box is connected to the back wall.
11. The twelfth connection is the back wall is connected to the flame box.
12. The thirteenth connection is the flame rod connected to the flame box.

I liken all of these connections as a Category 1 hurricane, and the amperage of the flame rod a fart! How is that tiny current supposed to deal with all of those connections? So I ran a ground wire from the switch box right to the flame rod screw (last and second to last pictures) bypassing 9 connections. My car mechanic says this is normal thing to do in service manuals in cars where various electronic problems show up in cars. I haven't seen it posted anywhere in my searches on the internet, but I am telling ya it works!

Old 01-27-21, 06:46 PM
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Welcome to the forums.

I've had this Carrier Weathermaker 9200 furnace for 15 years.....
Pretty much this is a new furnace
Not sure how 15 years equates to new...... regardless of what it looks like.
I work on these problems all the time.
Code 33 is not the flame sensor..... it's the flame rollout switch or high limit.

Just to touch on the flame sensor concept..... it's extremely reliable and for the most part easy to troubleshoot if you know how it works. I agree it's a very low DC current but that's all flame rectification can yield. The AC voltage is applied to the rod. As the AC voltage passes thru the flame it gets rectified into DC and a current loop is formed to the burner which is supposed to be grounded. The problem: it's not always well grounded.

The burner MUST be at the same ground potential as the control board or it will not pass current correctly.
Typically tightening all the screws and adding star washers on the burner and control board will correct the problem.

If it's a real problem system.... a ground wire from the burner to the metal frame and then to the control box will rectify the situation.


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