P2a00 -mercedes c220cdi engine management light on, obd2 reader showing this code

Question

2004 mercedes c220cdi estate, codes PO299
And p2a00, engine management light is on
Please advise what these codes mean? The
Automatic transmission kick down sometimes
Works and the vehicle sometimes does not drop
A gear on acceleration. The vehicle has done 122000 miles, the transmission fluid has never
Been changed but is showing full on the dipstick
The fluid colour is black & I believe it should be red?