The camera was a 60d, but in fact could be a 40d, 50d etc. – any camera with two motors.
Symptom: mirror resting in upper position. when pressing the release button, mirror went down quick, then up an err20.
I suspected that the phase sensor gives a non real position. I disassembled the mirror gear.
On he flex cable the sectors give the position through a metallic brush. I observed some dirt between two sectors. I checked with a multimeter, in resistance mode (NOT continuity).
Instead of the normal infinite resistance, I have measured this:
Ouuuups. This could be the problem. I cleaned it, after that:
This is the normal value. Be careful. As the phase sensor goes into a comparator, should not be a real short. Even a 1-2 kiloohm can be interpreted by the processióor, as a high or low level signal, as this contacts are connected to a voltage divider.
I assembled the camera – and it’s perfect.
The camera has been checked in another service too: they wanted to change the whole mirror box…