Sonic vi, I should change my signature really, I now have a Mk4 Golf, but your advise still applies as it is an AUM engine which mechanicaly inluding the K03s turbo is identical to the AUQ fitted to the LC! The only change is the ECU is mapped to produce 150bhp, not 180bhp.
I have been suspicious of the maf, but it is peaking at 125g/s which I beleive is correct for a 150bhp 1.8t. Also this fault is intermittent, the car can run for a fiew days and not show any faults, then it pops back up again.
There are no other fault codes coming up, the diverter valve is fine, as are the hoses going to it. I've inspected the hoses going to the N249 valve too, none of those show signs of damage, in fact when I removed the N249 for an inspection, the hose I first disconnected was holding a vacuum.
One thing I did notice is the black plastic breather cap on the N249 was quite tight, I removed it, and refitted it which has allowed it to move around much more freely. I did this on Saturday, last night there were no faults showing.