Downtown Line (DTL) operator SBS Transit has fixed the last malfunctioning door at Botanic Gardens station, but it still does not know what caused the glitch.
SBS Transit spokesman Tammy Tan said a faulty cable and connector had been replaced.
The affected components have been sent for investigations.
The fault, which was discovered at 5.45am on Wednesday, hampered signalling communication from the trains, and rendered all platform doors on the southbound side of the Botanic Gardens station inoperable when trains pulled in.
Station platform doors are synchronised with the train's signalling system. A two-way communication lets the doors open when the train has come to a full stop; and the train will move off only when all doors are closed. On Wednesday morning, station staff had to open and close the doors manually, by pushing a button. This led to slower train services during the morning peak.
At around 9.40am, SBS Transit decided to have trains bypass the Botanic Gardens station, so as to fix the problem. Four hours later, it managed to fix all but two doors. One was fixed on Thursday, and the last one, before service started yesterday.
Operator SBS Transit, a subsidiary of transport giant ComfortDelGro Corp, said in a Facebook post yesterday that it was "looking into the cause of the incident" with the Land Transport Authority (LTA).
An LTA spokesman said the cause has not been ascertained and will require further investigation.
The doors on the Downtown Line were supplied by Faiveley Transport, a French company which has an engineering joint venture with SMRT Corp. Faiveley does not supply doors to any other rail line here.
Wednesday's incident was the third known door-related fault on the DTL in the past seven months.
On Oct 4 last year, a dislodged platform door at the line's Sixth Avenue station disrupted service for two hours. The fault was traced to a loose bolt. On Feb 2 this year, a door at the Bugis station was cordoned off because of cracks. The latest flaw is estimated to have affected at least 10,000 commuters.
Experts said disruptions caused by platform door glitches are relatively rare, although some doors failed to open during recent test-runs of the North-South Line's new signalling system. ST understands the bug, related to software, has since been ironed out.
Singapore Institute of Technology assistant professor Andrew Ng said the recent glitch may lie with control units in the platform doors.
Prof Ng said: "To identify the root cause of the problem can be challenging because platform screen doors are composed of mechanical and electrical parts, and their operation involves various subsystems, with hardware and software components."