Upon privatisation of Britain's railways, the Class 121 fleet was only operated by one passenger company, namely Silverlink, with several more units in departmental duties with Railtrack.

55023/27/29/31, were based at Bletchley depot, where staff repainted set L123 (55023) into its original British Railways green livery.

The unit has been repainted into Arriva colours and was hauled to Wales in June 2006 to enter service in July.

The unit finally entered service on 16 August 2006 but was withdrawn 3 days later due to a major problem with the engine.

In March 2013, following further extensive periods out of traffic, 121032 was finally withdrawn from service, due to a terminal engine fault.

In 2003, Chiltern Railways reintroduced "Heritage" diesel multiple units on its Aylesbury to Princes Risborough shuttle service.

For this purpose, unit 121020 was purchased from Network Rail, and heavily refurbished to allow it to operate passenger services.

This unit has been fitted with secondary (magnetic) door locking and other safety features, and thus was exempt from 30 November 2005 deadline for the withdrawal of all Mark 1 vehicles.

Electronic destination indicators and internal passenger information systems were recovered from "Network Turbo" 165 032 at refurbishment and fitted to this unit.

Unit 121034, previously based at Tyseley Loco Works, was (less extensively) refurbished for use by the Birmingham Railway Museum.

[14] Both units were additionally used on shuttles, as required, between Aylesbury and Quainton Road stations, on the occasion of events at the Buckinghamshire Railway Centre.