Flame sensor is good

[Core_d]

does it have diagnostic light?

[Ac_man]

Thanks for the info

[Mr Bill]

does it have diagnostic light?

Can't always count on them, the Ruud unit I repaired was flashing a pressure switch code.

[Core_d]

I would start by checking from your "neutrals" on the board to any place on the chassis that is a good ground, make sure the unit is calling for heat. Also read it from the flame sensor to chassis and then to neutral on board, again make sure the unit is calling for heat. your going to read a .03 volts, maybe a little more or less, anymore you have a grounding issue.

Thank you mister bill. I like that answer im gonna mess around with this next lab day at scool loosen wires and what not

[Milk man]

What is the Model number of the furnace?

[stay gold jonny]

The wire from the sensor to the molex plug on the board was not making a good connection, I have a tool to take out the pins in the molex plugs, and I just pulled the old pin out, cut it off and installed a new pin and it was good to go.

What is the tool you have, and where can it be purchased?

[Bsmith816]

Did u put a manometer on to see what the differential pressure is? Checking for debris doesn't mean anything

[tkhvacguy]

I had a older Lennox unit doing the same thing a couple weeks ago what I found was a weak inducer motor that when the burners light up would cause the pressure switch to open briefly but never did get an error code just kept trying to relight without locking out

[socotech]

I've had a goodman do the exact same thing to me as post #28.

[DeHeatify]

I had a older Lennox unit doing the same thing a couple weeks ago what I found was a weak inducer motor that when the burners light up would cause the pressure switch to open briefly but never did get an error code just kept trying to relight without locking out

I had one do this to me too. I cleaned everything and left it working and tested x5 and the next day the on-call guy (seasoned tech) found the p/s fluttering.

how did you find out it was a weak inducer? pressure draw?

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[captube]

What is the tool you have, and where can it be purchased?

Probably talking about a molex pin extractor, Radio Shack or google.

[GastecD]

You could have a crack in the porcelain around the flame sensor that once heated expands and grounds out

[tkhvacguy]

I had one do this to me too. I cleaned everything and left it working and tested x5 and the next day the on-call guy (seasoned tech) found the p/s fluttering.

how did you find out it was a weak inducer? pressure draw?

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Yes used my manometer to get the pressure draw on the pressure switch

[Mr Bill]

Probably talking about a molex pin extractor, Radio Shack or google.

Correct! we have a great little electronics supply house, not far from me that has everything, you could build the space shuttle with all that's in that store. I buy all my solderless terminals and etc. in there, they have bigger bulk packages and cost less than my supply house.

[Mr Bill]

You could have a crack in the porcelain around the flame sensor that once heated expands and grounds out

Could you eloborate a little on how this would happen? Thanks!

[Mr Bill]

I had a older Lennox unit doing the same thing a couple weeks ago what I found was a weak inducer motor that when the burners light up would cause the pressure switch to open briefly but never did get an error code just kept trying to relight without locking out

This is why you carry plenty jumpers, you jump one safety at a time till you find the issue. I always check/jump all the safety's first then once they are ruled out I move on.

[wdshea]

you can check your ground by checking continuity between neutral and ground. Ground and neutral are electrically connected (bonded) inside the breaker box. though i don't know if the unit cares about earth ground or just ground as a reference. carrier support told me that their units need a good earth ground even if ground reference is good. don't ask me why.

[Mr Bill]

you can check your ground by checking continuity between neutral and ground.

Exactly! this is why I mentioned to check from the neutrals on the board to a cabinet ground, if I suspect an issue, I always jumper from the neutral/neutrals on the board to the cabinet ground.

[timebuilder]

The process of ground checking in this situation is to eliminate any high resistance points between the board/module and the burners. The burners are the negative side of the rectification circuit, and the flame sense rod is the positive side. The board or module supplies an AC voltage to the sensor rod, referenced to the burner ground, and then it looks for the DC microamp current, as a result of the flame rectification.

You MUST have a good connection between the burners and the ground connection on the board or module. In rooftop units, you can apply a jumper from the burner array to the module, and if the problem goes away, you know you have a ground connection issue.

[Mr Bill]

You MUST have a good connection between the burners and the ground connection on the board or module.

Amen Brother Ben, shot at the fox and killed the hen.