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Well I can tell you one thing you definitely got ripped off at $537.00 in the first place, the timer and defrost thermostat is the total of $68.00 so I'd say he did pretty well charging you over $450.00 for labor, Wow. At any rate if you really did replace them to begin with one of them could be bad again that is possible but unlikely, this job should've cost you around $160.00 tops so you did get ripped off, what I would do is call around and get some service prices from different companies stay away from that guy. Tell them only that you had the defrost timer in defrost thermostat replaced but don't tell them how much it cost you, just say you didn't like the guy and would rather somebody else come out and look at it give them the model number and asked them what they would charge to replace those two parts if it needed them, you can do this yourself but it's best to test them before ordering them and replacing them, if you wish to do it yourself you would need a voltmeter and know how to check continuity in order to see where the problem lies, let me know what you want to do, like I said most places charge around $160.00 for that whole job with both the parts, and even if it was the defrost heater that's only $28.00, thanks and well wishes, Bryan