It is taking RPM readings from number four injector, usually revs at around 2000 rpm if it is doing that, because it only gets a signal every two revolutions. As said before DO NOT remove bracket that the sensor bolts to. ONLY remove the sensor from the bracket.
Air gap 1 mm is set with a special tool to set bracket in correct position. That is why i stressed NOT to remove bracket. No washer that effects sensor position that i know of. Unless someone has used a very innovative way of gaping the sensor. Bolt sensor to bracket, slide sensor and bracket in until sensor touches timing pip. Bolt bracket down then remove sensor and use a 1 mm thick washer to space sensor back from bracket. Pretty ingenious way of doing it if you don't have LRT-12-122 handy.