From a TSB on this (note it says you have to do it twice):

The MY 1996, 1997 and 1998 Volvo 850/S70/V70/C70s require an extended driving period in order to reset readiness codes from INCOMPLETE to COMPLETE. When verifying an emission related repair, the TRIP identified in Section KKKK of the Engine Management System Service Manual TP 2308202 M 4.4 or Section EBU of the Engine Management System Service Manual TP 2301202 M 4.3 may not allow all of the diagnostic monitors to run.

The following instructions, which supersede TRIP instructions contained in the above referenced service manual, will exercise all of the diagnostic monitors on these models. As long as no diagnostic trouble codes have been stored, the readiness codes will be set to COMPLETE upon completion of two consecutive trips driven according to the instructions.

Introduction 1

This on-road driving sequence will allow all of the diagnostic functions an opportunity to run. After two correct, consecutive TRIPS, the readiness codes should be set to COMPLETE, provided no diagnostic trouble codes have been stored.

General Instructions 2

Engine temperature at start must be below +50°C (122°F).

Air conditioning (A/C) must be off during idle periods. A/C compressor activation during the Evap diagnostic may interrupt the Evap test.

Select a suitable route where it is possible to safely stop at the side of the road. Obey all traffic regulations and posted speed limits.

Trip Instructions 3

^ Start engine

^ Move gear selector to Drive

^ Accelerate gently to 1500 - 2000 rpm

^ Drive for 6 minutes at 1500 - 2000 rpm

^ Safely stop the car and idle engine for 70 seconds with Drive engaged

^ Accelerate to 1600 - 2000 rpm

^ Drive for 6 minutes at 1600 - 2000 rpm

^ Stop the car and idle engine for 70 seconds with drive engaged

^ Accelerate to at least 45 mph, drive for 5 minutes at 1800 - 2200 rpm

^ Stop the car and idle engine for 60 seconds with Drive engaged

^ Accelerate to at least 45 mph, drive for 4 minutes at 1800 - 2200 rpm

^ Stop the car and idle engine for 60 seconds with Drive engaged

^ Drive the car to the location where it will be parked

^ Move gear selector to Park

^ Stop engine Finished