crank sensor is expensive, so wouldnt surprise me if bulb has been removed to fool the next owner.
thing is, even with a faulty crank, it can idle fine, and can start fine. the problems occur when its warmed up, and these can obviously be intermittent. i gave mine a clean, but it made no difference, as soon as i fitted a new one, it was immediately different. started better, idled better, ran MUCH better and has never stalled or anything since. worth a thought, just be aware, new, they are approx £200 +- £10 depends where you go, and id avoid used, i only say this becaus ei tried two used ones, neither worked.