Intermittent start fault. Ecm light on but no fault codes

I have an intermittent fault on my '02 Td4. When it feels like, my ecm warning light illuminates the engine will turn over but refuses to fire up. I've checked with an OBD2 reader and no fault codes are present. Reading through the previous threads I've checked all the sensors and they're all working fine as I've swapped them over on to my'03 td4 with no problems.
When the ecm light extinguishes(which could be an hour or a few days later) the bloody thing will start as if nothing had happened. It makes no difference if the engine is hot or cold either!!
Any ideas anybody??

How many fuel pumps have you got.
If you have got one under the bonnet, make sure it is running as well as the one in the tank.
You need both for the engine to start.
You should be able to hear both for about 30 seconds with the ignition on.
My under bonnet pump stopped working leaving me stranded the other week.
I cleaned the connector and all seems well(at the moment)
Still got the engine management light on though.
Can`t clear it with an OBD 11 tool.