@:>,.; mot

[RogerHeinz57]

Yes, a strange phenomenon and no specific answer as such, but one tester may consider the aim to be within limits and another finds an error of the previous adjustment, a bulb change may cause the bulb holder to become inadvertently shifted, the rough roads could also be a cause if adjusters to become shifted/broken.
These are only theories.

The quest for zero advisories on our age of car at MOT are now limited to only a few realistically.

[crazy frog]

Originally Posted by Comfortably Numb

How does a headlight move out of correct alignment, unless through damage or removal for some other purpose? In which case, would you not have the alignment checked?

Many different reasons for up/down only adjustment. Some have already been mentioned. Never had a left/right adjustment except after changing my xenon bulbs. Pig of a job bumper off full headlight unit out ballast pack off just to change the bulb.

Heres a few I know about

1. Compare an unladen car. Brand new over inflated rear tyres new rear springs empty boot, 1 Gallon of fuel, 5 litres of screen wash, lazy high mileage front springs (but legal) worn front under inflated tyres (again still legal) or even replacement lower rated front springs that make the front sit lower to start with. All the above making the nose angle sit lower. Headlights adjusted and a PASS.

So to next years MOT it now has all the opposite.

2. Worn rear under inflated tyres, lazy high mileage rear springs,a boot full of tools/gadgets/ small trolly jack an axle stand fire extinguisher etc. etc. 15 gallons of fuel 1/2 litre screen wash. New over inflated front tyres and new front springs. All the above making the nose angle sit higher and it FAILS because the beam is to high.( for good measure you could fit a tow bar just to exaggerate the point)

Now the above are both extremes, but it only takes a few degrees of angle at source to change a light beams path at the required distance. and the above examples could easily gain or loose 3/4 inch height, therefor affecting the angle.

I'm not advocating any of the above.
But Auto levelling headlights should sort most of the above, but not all cars have them.

A related issue is wheel Alignment /caster/ camber toe ,especially 4 wheel. Where the correct weight distribution / amount of fuel/ car balance/ tyre condition and pressures should all be within certain parameters whilst checking ,and after every adjustment driving back and forth a few metres to allow the wheels to settle before further measurements are taken.

[RoverP480]

I had exactly the above. Fitted new rear suspension arms on my Discovery before MoT .and it failed on Headlamp beam , too low. Although it self levels, these arms have large rubber bushes and it pushed the back end up a bit. As described it was a fail followed immediately by a pass as the garage adjusted them, FOC, which I appreciated.