It was taken out of service just under a fortnight ago after encountering a power problem at Castletown.
And it has emerged that the green machine has only been operating on three out of its four traction motors since it re-entered service.
Loco no.21 had been languishing in the goods shed at Castletown ever since it was taken out of service as a precautionary measure on October 25 after it experienced abnormally high power consumption on one of its traction motors.
Engineers at Isle of Man Transport waited for the season to end before taking the diesel slowly back to Douglas under its own power yesterday (Wednesday).
They will be carrying out tests to identify the cause of the fault.
Diesel no.21, unkindly dubbed The Cabbage, had only re-entered passenger service at the end of August following major repairs, having previously been out of action since June 2015.
The loco was purchased at a cost of £420,000 in 2013 to pull commuter, dining and maintenance trains as well as acting as a shunter and recovery loco.
But it has been dogged with technical problems ever since it was first delivered.
Early tests revealed a number of teething problems including overheating, and its engine was replaced by the manufacturer’s under warranty. Issues were subsequently with the bogies, which are over 40 years old and not covered by the warranty.
They were sent to the UK for tests and repair.
Some £246,800 has been spent on getting the diesel fixed.
Following commissioning tests, it finally returned to traffic on August 22.
But it has been revealed that it has been working on three out of four traction motors, following problems with the fourth, although this makes no difference to its performance.
Abnormal
A spokesman for Isle of Man Transport told the Manx Independent: ’On Friday October 25, we experienced an abnormal current draw on one of the traction motors and it was decided to withdraw the loco from service as a precautionary measure.
’The loco has been returned to the workshops as planned but there were no further instances of the abnormal current draw experienced during the return trip, which was undertaken without incident.
’During the commissioning phase one of the four motors performed poorly on insulation tests and was isolated, again to prevent the possibility of any damage being inflicted on the motor.’
The spokesman added: ’The loco has enough redundancy to allow the disconnection of a motor.
’We will undertake further investigation and any necessary repairs, and the loco will be returned to service.’