Volvo Repair: Check engine light, oil sludge, variable valve timing

Question
I have a 2001 volvo with a 1.9t. The check engine lights on constantly. I've had it checked out,they say it's one of 3 things,oil sludge,timming belt,or camshaft sensor. It runs good except at times it idles rough or misses. which would be the most common fix?

Answer
  Daniel, they are really coverring their bases aren't they? VBG I would like to know what the code is, but they are probably talking about the variable valve timing. The variable actuator is hydraulic (oil fed) and sludge will plug the passages and affect function. Most common. If it was the belt, it would do it all the time. An intermittent cam sensor could also be the problem. Any guy worth his water should be able to watch the cam sensor output on a scope, when it's acting up,  and determine if it's good.
  the next step would be to drain the oil and filter, pull the actuator out and clean it's passages. Reinstall, run for 15-20c minutes and change the oil again. I DO NOT LIKE additives. So use at your own risk. HTH

Mark