When I start Heidi (well I am in Switzerland) the ABS light remains on until I drive away ...well normally. Sometimes it remains on, that is unless I keep my foot on the brake when I start the car. If I do this the ABS light goes off as normal. What causes this strange behavior? I have been advised to give the ABS sensors a sharp tap with a hammer.

The behavior started after a rugged trip to the Sahara.