Hi, I have an mot booked in and the warning light has come on for the 3rd brake light.

I checked and it was out, but a tap and it comes back on.

Would an mot tester tap it? (if I mentioned it to them) ? if it was out during testing?

Not sure it is a bulb unit replacement as it seems to work when tapped, so may be a faulty connection somewhere?

 

If fails - how do I fix it? or look at tracing the fault?

Not sure how much this helps, but if it's item 11, there are 3 nuts on the inside that you need to access and undo to investigate: https://skoda.7zap.com/en/cz/octavia/oct/2008-419/9/943-943000/.  You'll probably find some corroded contacts where the wiring meets the light cluster.

I picked up the same snag on mine on a pre test check.

 

Whip the tailgate inner panels off, 3 nuts securing the high level led assembly and disconnect the wiring.

 

The connectors were corroded/green so I applied a bit of T-Cut and (dis)connected them a few times to clean things up before a good rinsing with WD-40.

 

Test and reassemble.

 

I wouldn't torque the plastic nuts more than finger snug as the plastic threaded protrusion will shear off.

Thanks. Will give a go on weekend

OK, so managed to get a look at this,

the rear boot trim is indeed 2 parts. the screw under the pull handle, and pull around edges to remove the lower section, this then reveals 2 more screws (one each side) for the top section, removed that to access the light.

Couldn't see any corrosion, looks pretty clean on the contacts. which is annoying, as now not sure what would cause intermittent fault, looks like will have to replace the unit, to rule that out. £40 I don't have right now.

 

You should check the connection is getting power on brake application before buying another unit.

 

Test using a meter or even a 12v bulb, 501 sidelight bulbs have a bare wire for easy poking.

 

You could also test the unit separately using a 9v battery, DC PSU or wired to car battery. Remember it is polarity sensitive.