Tesco's yes but 3 weeks ago.......did you buy tesco or morrisons petrol?
Water levels are fine, it could well be the Tesco's petrol but thought it would have raised its ugly head by now being 3-4 weeks later......hows you water level?
The price of oxegen sensors are through the roof (that's if you can get, or need one) If it is through silicone , I hope you have kept your petrol receipt. Supermarket sh!te