If you believe what's presented in tech manuals...ie...."Automotive Air Conditioning Basic Service Training Manual" by Mastercool and what I think I've read here, it's possible the system is indeed overcharged.
There's a pressure-to-temperature relationship - refrigerant. Ideally, you want things close to but not quite at 32F in the evaporator. I'm not sure why the tech would be looking for 40lbs on the low side. That equates to 45F. Between 20-80 psi, temp. & pressure change together - 1 psi increase in pressure results in 1 degree F change in temp. Ideally, you'd want a low side pressure closer to about 30 psi. I find it odd that the high side was not checked at the same time?
Is the system cycling more often than when it was performing correctly? Overcharged systems cycle often and vents temps. are less than desireable.
As Steve Brothertom pointed out, your compressor could be sick and in this case, I'd expect to see low pressure on the high side.
Bottom line - tech didn't read both sides, so it's tough to say. In my opinion, you should have it reevaluated by someone else.
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