2000 Volvo S80

1st step should be getting codes
SELF-DIAGNOSTIC SYSTEMS
ON-BOARD DIAGNOSTICS
This system has complete self-diagnostic capabilities. This systems uses a single ECM that receives input from engine monitoring sensors. ECM uses these input signals to control air/fuel mixture for emission control, fuel economy and good driveability.
Signals from various sensors are monitored continuously. If certain signals are lost or become faulty, ECM will adopt fixed substitute values (limp-home mode) to enable vehicle to be driven for certain failures. Substitute values can be adopted for Engine Coolant Temperature sensor signal, Throttle Position sensor signal, Mass Airflow sensor signal, and oxygen sensor signal.
NOTE:To avoid confusion if there is an intermittent malfunction with Malfunction Indicator Light (MIL), carefully read following text.

Faults are recorded in ECM memory in the form of Diagnostic Trouble Codes (DTCs). Codes can be displayed using a generic scan tester connected to On-Board Diagnostics II (OBD-II) Data Link Connector (DLC).
ECM stores Diagnostic Trouble Codes (DTCs) differently in accordance with a predetermined pattern. An emission related fault must be present for a predetermined number of intervals before ECM permanently stores a DTC and illuminates MIL. If fault is cleared before a specified interval has passed, DTC is erased from memory.