This design was arguably the least successful diesel locomotive ever employed on British Railways after the ill-fated Co-Bo. The twin Paxman engines were unreliable, being prone to camshaft and cylinder head problems amongst others;[4]:58 overall availability was around 60%, even after extensive modifications. Forward visibility, which had dictated the whole design of the type, was not as good as had been hoped, the long noses meaning that the crew could not see the area immediately in front of the locomotive. Although the Rolls Royce and Crompton Parkinson engined examples had better reliability, no further examples were ordered and BR decided to dispose of the type, replacing them with an order for 100 of the already proven Class 20 locomotives.