Crazy Marty

I have had the following codes show up intermittently on my 1998 Audi A6 2.8L 30V Quattro and would like to finally do something about it.

16711 P0327 Knock Sensor 1 (G61): Signal too Low
16716 P0332 Knock Sensor 2 (G66): Signal too Low

17545 P1137 Fuel Trim: Bank 1 (Add): System too Rich
17547 P1139 Fuel Trim: Bank 2 (Add): System too Rich

Sometimes they don't set the MIL light off for weeks / months.
Sometime, like this week, it has set the MIL light off several times.

I live near Vancouver, Canada and wonder if the colder weather from Sept to March is playing a role.

The Bentley CD says for Knock Sensor:
Check knock sensors and knock sensor control =>

For System too Rich, is says:
Check Heated Oxygen Sensor (HO2S) -G39- (bank 1, sensor 1) and O2S control =>
Check fuel pressure, fuel pressure regulator and residual pressure =>
Check exhaust system for leakage

Any other thoughts???