How the fix err01

https://support.usa.canon.com/kb/index?p...

Err 01 is a lens error in most cases; I've never seen a 5D/6D or 2-digit prosumer camera where the issue lay with the body (outside of the APS-C 70D where Canon acknowledged the issue), it always seems to be bad lenses (the entry-level bodies on the other hand... yeah it can very well be the body). How you can usually tell to really quickly to isolate the camera from the lens is do two tests:

• PUT THE BODY CAP ON and fire it a few times. If it fires without a lens, then it's due to a lens failure. You do not want to do this with a open sensor unless you have a sensor cleaning kit!
• Test the camera with a cheap EF lens; you can't use the god-awful 18-55 kit lens we do not like on the APS-C side to test for this on the full-frame bodies. Fire it a few times and see. Again, no error indicates a lens issue; likely the electronics inside the lens. If it fails with two lenses and the "tester" is known good, the body may be suspect. I know you can sometimes get chipped M42 adapters to "fake" having a lens, but you need to use a real lens (genuine Canon favored, but modern lenses from companies like Sigma work) to test with full communication. It's the ONLY WAY to absolutely rule out the body; these stimulate a lens like a 50mm and open up the f stops which makes them poor candidates. Chipped M42 is fine for adapting vintage glass, NOT TESTING FAULTS WITH EDGE ON BEING MAJOR.

Start there and go from there. If it's the body, the problem lies with the lens contact PCB assembly (it's under the pogo pin assembly used to interface with your lenses). You pretty much have to take most of the camera apart to get to it, so either see if you can find the 5DII SM or a tutorial and assess if you can do it or send it out to a shop with a reasonable hourly rate. This isn't an easy fix anyone can do.

That said before you go through with the repair, see if you can check the shutter count with EOSInfo or ShutterCheck; do not try and repair this fault if the shutter count is well into the 100k range as these only have a ~150k mech shutter. If it has 20-30k I'd go for it; but with anything in the 100k range (even 101k) due to the risks of losing the camera again and needing to do a $250-300 professional repair (or suffer taking the camera apart myself, ordering a shutter assembly and swapping it and praying I got the sensor alignment right) is a risk due to the high count. The camera might last 250k clicks, but do you want to find out you spent a few hundred to fix a major issue to need a shutter 10k clicks later? Unless it's something like a 1D series which can be as high as 400k on the 1DX, don't.

As far as replacement goes, this can go both ways if you have a lot of Canon glass: I would look for a 5DIII (NOTE: You lose the swappable focusing screen but it beats replacing the shutter early) with a realistic click count; skip anything the seller will not disclose, it's high on a camera like this if they hide it or go for a Canon RF Mirrorless camera and buy the EF>RF adapter ($100).