A Malaysia Airlines Boeing 737-800, registration 9M-MLS performing flight MH-1471 from Langkawi to Kuala Lumpur (Malaysia), was climbing through FL200 out of Langkawi when the crew initiated an emergency descent to FL100 due to the cabin not pressurizing properly, the passenger oxygen masks did not deploy. The aircraft continued to Kuala Lumpur on FL100 and landed safely on Kuala Lumpur's runway 32L about one hour later. A passenger reported the captain announced "emergency descent" following a violent pitch down. The passenger noticed that the cabin did not cool down after engine start. The aircraft resumed service after about 9 hours on the ground in Kuala Lumpur.