Interesting - I've seen the switch on some older cars, but didn't realise thats what its for. The casting where the heater valve attaches to the heads retains the threads to hold that switch even on my S4's.

In other news - I compared the AFR with the euro air pump adding air, and it changes the mixture by 2. When idle mixture is set to stoich (14.7) and I then reconnect vacuum to the diverter valve, AFR jumps to 16.8.

So when I'd set the mixture before without disconnecting the air pump's vacuum, I'd set it *way* too rich!