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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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What does it mean when the red light (DEF) comes on the

Customer Question

What does it mean when the red light (DEF) comes on the machine
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: just tiday, the system wont operate on auto
JA: Do you plan on doing the work yourself?
Customer: what work?
JA: Anything else we should know to help you best?
Customer: no
Submitted: 1 month ago.
Category: HVAC
Expert:  Phil replied 1 month ago.

Hello,I am a retired mechanical engineer with 56 years in this business. I will go in depth with you, and up to a 6 hour time limit.

All I ask is please select the ***RATING LINK*** at the top of your page before leaving, that is the only way I am credited through the site.

Rating does not affect your charges nor ability to refund if not satisfied. If you can agree to that, lets continue by all means.


DEF usually means 'defrost' and it applies to heat pump type AC systems, if you can give me the brand and model number of the unit that sits outside I can see what you have, we can go from there in 6 hours, I am off to bed its late here.

There are no other HVAC experts on line here tonight apparently. However if you want faster service there is a chance that someone else will log on and help you before I get that case just opt me out so that your question goes back up on the board.

If you work with me you will need a multi meter and may need to send me a photo of the unit wiring diagram if the manual for your system is not on line.