The ignition timing sensor could be an issue, along with a host of other potential issues. The bottom line is you're going to need an experienced smog technician to diagnose your Volvo's smog check failure all the way to smog check passing status.
Some times finding the root cause of high emissions can be difficult. On older vehicles with emission components ready to quit, even harder. The smog tech ends up replacing smog parts which appear and for the most part are defective, only to find additional barely functioning sensors and/or solenoids which need replacement also. Often it is not apparent a certain component requires replacement until after a pre-component has been replaced/repaired. Passing the emissions test requires all the smog components on your Volvo 940 be working together and properly.
