faulty granada onboard computer

can any one help with my faulty onboard computer in my granada scopio cosworth, clock works fine, the miles left to drive on fuel in tank works fine, but the MPG just displays Zero, I recon it must be a faulty sensor
somewhere but where and which one. Any ideas welcome
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I think that sensor is in the metering block that returns unused fuel back to the tank, now if I could think where it was,,,,,,,,,,,,,,
m
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