Help! 2001 WS6 does not want to start

My car for the last 2 months is having a problem starting. The problem is intermittent. The car turns over and cranks no problem. I have tried using
my spare key in case the original was causing the antitheft system to trip (resistor in key) but it still is doing it. In all cases the car has started within 15 minutes with times of a few seconds up to the 15 minutes. If it takes a long time there is a smell of exhaust gas for a couple of seconds. When it does start it catches immediatly no sputtering or backfiring. I am thinking some part of the electrical system may be wet/poor connection.
Has anyone run into this before or have an idea what may cause it. The car has gone in twice for this and both times it would not start for them but when they started doing diagnostics it started and the problem would not repeat. I also think he said they checked fuel pressure at the rails and it was good.
Thanks in advance
Richard Taylor

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