Is it just the traction light or traction and serve light?
What position is the steering wheel in why your going straight? I.e is the tracking out?
I had a similar problem when I had fitted a load of bits and tracking was out, I knew the cause so I tracked it and it was sweet. Now this may apply to your car or may not as some ph2 172's were fitted with speed sensors and some work the speed out via abs.
Now if the steering angle sensor is measuring more the 5 degree yet it's still going in a straight line (tracking out) it gets all confused and chucks the light on. This is cause it doesn't notice the difference in the front wheel speeds that it should when cornering like it should, hence the light coming on.
Only if your wheel is in the position described lol