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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8437
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have error 67 and 91 and Ac not working - help!!

Customer Question

Hi I have error 67 and 91 and Ac not working - help!!
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.

Hello. I am sorry that we are late, it seems that neither I or anyone else here could find the error codes you mentioned.

Any time I see two error codes it is about about 60% likely that the printed circuit board that sends out the error codes is bad... not always...but many times. We can do a heat stress test on the printed circuit card where the thermostat wires attach inside the blower compartment of the furnace if you like.

Use a hair drier to slowly and gently warm the card to 120F (warm to the touch but not hot and not over140F)... if you get any change in behavior that means the card is bad...

A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.

Constant expansion and contraction as these electronics hea tand during their duty cycles over the years is what wears these cards out.

It is also a good idea to re-plug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

These cards are also damaged by Electrical Spikes caused by power failures and lightning. These high voltage spikes can literally explode the fine 'wire' connections inside the chip.There is also a shunt type device, often printed on to the card that allows a high voltage spike to arc across to ground harmlessly, some of these can wear out after a few times and allow the high voltage spike into the rest of the card and the CPU micro-circuits. The high end printed circuit cards use a large capacitor like device, or several in parallel, to capture and release such surges, these do not wear out.

Let me know what you can discover, we can go from there if you like without a time limit.

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