Found this while doing service with a good friend on his loved ones car (Peugeot 107 1.0, same platform as Toyota Aygo) : The engine is a Toyota 1KR-FE, 3 cylinder NA petrol, 998ccm. 130000 Km. the Spark plugs (Denso K16HR-U11) seems to be from the factory. They were rather hard (but not doing noises while unscrewing) to unscrew. They were only causing misfires at full load or so. No starting issues or whatever, I was really surprised with a gap close to 2mm ! We fitted NGK LFR6C-11 as a replacement with a line of copper antiseize paste on the threads. So far so good.