Update on the climate control acting up post.
The Engine Auxiliary Water Pump, (AKA heater coolant pump, coolant pump, etc) is not running.
2006 XC90 4.4L, all options except Nav., 180k miles, runs...

The temperature just dropped like a rock in the Dallas area 01/02/2002. And my 06 XC90 4.4l isn't blowing hot air in the cabin. If I fiddle with the controls for a while: change vents, change fans...

Fixed. After the last drive test failed with the same codes, I was at a loss to understand this problem. All vacuum leaks were repaired. Vehicle runs perfect. Same coded. Then I stumbled on a...

2006 Volvo XC90, 4.4L, AWD, 181k miles. Starts, runs, drives very good, even with the codes.

I am asking for help/advice in solving this nagging problem: P0174 current fault, P0171 pending...