These days with all the hassle to get at them to fit, it makes good sense to fit the set.

On the 2010 Fiat Scudo 2.0 litre I had, I hoped it was the relay, but on testing it proved to be the heater plugs and this involved working at the back of the engine against the bulkhead removing the egr cooler etc. I used a endoscope camera I had luckily recently bought.