For Error 6, test the input circuit resistance (which I would do while it is powered up). I would first suspect the relays.
Self-Test flips the Front/Rear relays to the Front. There are two connectors on the rear backplane on the left side that come from the Front/Rear relay board. The one with two wires switch the V+ and I+ terminals and come from the 2-Pole relay. The one with four wires switch the V-, I-, V-Guard, and Ohms-Guard circuits and come from the 4-Pole relay. Pull these two connectors from the backplane, move the 2-Wire/4-Wire switch to 4-Wire and the Guard switch to Remote for testing purposes, set the Front/Rear switch to Front, and test the resistance of V+, V-, I+, I-, V-Guard, and Ohms-Guard from the front panel to the connectors (which will also test the relays). If the resistance is over 2 Ohms, then the relays need to be replaced or burnished.
I would also test the Ohms PCB relay. The pins that need to be jumpered with the Ohms PCB removed are shorted out by the Ohms PCB brown connector when the Ohms function is not selected.
During Self-Test, the 2-Wire/4-Wire switch needs to be set to 2-Wire and it also needs to be good or it can cause Error 6, so measure across it also.
For Error 7, make sure the Remote/Local Guard switch is set to Local.