TC reported TIMS showed rear M2 car 'B4'door did not open for all stations. Station staff(MSL) chec ked and confirmed 'B4' door at rear M2 car did not open at p latform. SM(WDL) checked and confirmed 'B4' door did not ope ~~~~~ Confirmed #2523-B4 door not opened. Isolated air supply to manually push door open with significant force required. Mating strips on both door leaves found with dirt accumulation. No signs of FOD in J-rail and door guide except for some compacted dirt in rail. Bi-parting mechanism checked ok. Door manifold checked no leak. Door resistance 7kgf ok. Checked EMV 106.5V when door commanded open-ok. Serviced door & function test 20x-ok. Door opening timing 2.7s, door closing timing 2.8s-ok. Checked by Lim J.H. 21/03/21.