Reset/calibration of accelerometer?

I seem to have a problem with the accelerometer. Lately my Iphone won't shift into landscape mode (all apps).

Horizontal lock is not set, the Phone has been reset both via iTunes and via settings and it hasn't been dropped.

In a flat position the Z-value reads -2.30024.. permantly and doesn't change when moved in an angle of +/- 45 degrees (read by Axelerom app). If moved further than 45 degrees the Z-value does start changing, but the highest value achieved remains below zero.

Is there a way to generally reset/callibrate the accelerometer - or will I need to hand it in for repair?

Appreciate any help on this.

Thanks