Took 3 days to diagnose a faulty relay...

Booked my car in for an engine light issue. Got 3 calls explaining they had not started work on it yet, then once it went into the workshop, they went silent. 3 days later I finally got a diagnosis which seems improbable and is not covered under motor plan. Since when does it take all day for a software upgrade (which I never requested) I started getting suspicious when the initial feedback said it was a fuel sensor, then while chatting he mentioned something about low voltage causing it only to get a confirmed diagnosis saying the adblue relay was faulty but they may need to replace the entire tank! The engine light only came on now after I had the car for a year. Its curious cos the adblue level is below a third and the warnings are coming up now. But for 14K to replace a relay which they say is "definately faulty" I'll try top it up and see. R41 000 for an adblue tank replacement is unthinkable. Being threatened with motorplan cancellation again when I've paid every time anyway just makes it worse. Next car wont be a mercedes at this rate. Why accept a booking when the car sits in the queue until 2pm.