The STH solenoid is inside the transmission, under the pan type cover on the front side, The control module is the ECM, located next to the fuel injection ECM inside the black box behind the right headlamp. Diagnosis involves testing resistance of the solenoid from the wiring at the control module, which is best carried out using a special "break out box" that plugs into the harness in place of the removed ECM. To access the STH, the suspension sub frame has to be lowered a bit to gain access, and the pan removed. Could be bad components or wire chafing somewhere. Only way to estimate cost is to get it properly diagnosed. I can tell you that the solenoid is probably around 60-75 dollars, and labor about 3-4 hours. The ECM could run 800-900, but is easily replaced in 2 minutes. I strongly urge you do NOT use Aamco or similar trans shops. Volvo dealer or reputeable Volve independent specialist are your best bet to get this properly diagnosed.