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I'm assuming that this is the diesel engine?
in which case if the engine has been run at all with the contaminated fuel then you may still have got away with it, depending on how long it was run for, and what percentage of petrol was in the diesel
did the fuel tnak have any diesel in in before the petrol / gasoline was pumped in? if so how much?
po code 50 isn't a recognised fault code, as they normally begin with a P and are 4 digit, where are you reading this code off from?
if you can describe the issue I'll try to help further