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The only certain way of finding out what is wrong is to get the error codes read. Has your garage told you what they are?
Some error codes are specific to Saab and are best read by a Saab specialist with Saab's own diagnostic computer called "Tech2".
Whereabouts are you? Someone may be able to recommend a specialist. There are a lot about.
But the 1.9 diesel is a widely used engine. It is also used in Vauxhalls, Fiats, and Alfa Romeos.
The exhaust particulate filter can block if you don't do regular longer journeys. But the "Tech2" can force the filter to clean itself. It will only need renewing if this fails.