I found why my engines sometimes at the start is running like there is a bad mix between gas ans oxygen it was not the sensors, it was the pressure regulator(hope it is the right word in english) this regulator located on injectors made the gaz (when to mucch at the start go back to gas thank) so if you ever have probles at the start or a rpm drop at 500 rpm it may be that part. 120.00 for it (in canada. hope this can help someone because I have replace all sensors(includid knock sensor) and I was always having problems Andre sorry for the english