I have been getting a P0020 code (with engine light on). I am trying to get an understanding of what is going on here. Everywhere I read that a P0020 is because of an open circuit in the exhaust cam actuator (magnet) circuit. Here is the sequence of events.

1. Removed both magnets, Drilled out rivets, and resealed them. Refitted.
2. P0020 appeared soon after (one day) - exhaust cam actuator circuit open circuit.
3. Removed both magnets. Checked for open circuit and resistance. OK.
4. Refitted magnets - swapped inlet for exhaust.
5. Still getting P0020 - exhaust cam.

My car has done 210,000km, has a slight rattle from front of the motor on a hot idle which possibly maybe timing chain/tensioner rattle. (personally, I think the supercharger is the culprit). But I read on many forums that timing chain stretch can make it throw a P0020 code.

But I have some questions about the "stretched chain theory".Are the camshaft actuators (magnets) also camshaft position sensors?
If yes, how can this be since the magnets/actuators are only two-wire. A magnetic, hall or optical position sensor nearly always has 3 wires. 5V+ve, 5V-ve & the sensor output.
If it is in fact NOT the position sensor, where is the cam position sensor on a M271?I will be more than happy to be proven wrong in my assumptions. I just need to get to the bottom of this issue.