Recently changed the spark plugs on my Nissan Xtrail nt30, now fuel efficiency has greatly deteriorated, on average, I'm now using 15 litres to do 120km. Where can I start looking at to solve this problem?.
I would start with what you did, not to say you did the repair wrong but if it only just started that is what I would do. Did you use the correct plugs(heat range)? Does the engine have a miss? What was the condition of your old plugs?