Tried starting 98 C5 yesterday, battery was completely dead. Charged overnight and had 12.84 volts measured at battery terminals. Reset key fobs and they locked, unlocked and opened the trunk. Lights, horn, radio and heater blower motor all work. The key slot is NOT vertical when ignition is off and key is out of ignition but is almost on the dot on the plastic to the ACC position. Tried starting car. All gauges go to zero and accessories turn off but starter will not engage.
Turning key to the off position still leaves radio on with driver door open. I suspect this is what caused the battery to drain, when it should be off it still supplies power to accessories.
Used a different ignition key to make sure that it simply did not read the resistor pellet wrong.
After removing key found that you can turn the ignition switch lock cylinder into all positions WITH NO KEY. Used the tip of the key and was able to turn the metal part of the switch where the key is inserted simply by twisting the metal lock cylinder clockwise.
Pulled codes and got the following codes:
BCM B2721 H, B2722 H, B2723 H C VATS codes
LDCM U1096 H
RDCM U1096 H
RFA U1255 H
Suspect ignition switch lock cylinder and/or the actual electrical portion behind the cylinder. Can the interlock cable to the shifter be causing the reason why the switch will not return completely to the OFF vertical key position?
Should I try the VATs relearn procedure (3 times ignition on then off then on (10 minutes at a time) as the battery was dead for some time or will this screw up things worse than they are now?
Can anyone shed light on the cause and fix for this problem?

 

Certainly sounds like the ignition switch is faulty. Not familiar with the C5 ignition but if the actual switch is rod actuated at the base of the steering column next to the firewall, then check it also.

If your key chain has a lot of keys on it, then that can wear out the ignition lock.

 

Ignition cylinder may need to be replaced. Do you have a factory service manual?