A Westjet Boeing 737-800, registration C-GWSV performing flight WS-1402 from Calgary,AB (Canada) to Phoenix,AZ (USA) with 175 people on board, was climbing through 12,000 feet out of Calgary when fumes and smoke were noticed on board, the flight crew received a left PACK TRIPP OFF indication. The crew worked the related checklists, reset the left air conditioning system and the smoke and fumes began to dissipate. The crew declared an emergency, stopped the climb at about 17,500 feet and decided to return to Calgary, burned off fuel for about an hour and landed safely back in Calgary about 80 minutes after departure.
The Canadian TSB reported maintenance performed several checks of the left air conditioning system and returned the aircraft to service. The aircraft was involved in a similiar occurrence on its very next flight the following day, see Incident: Westjet B738 at Calgary on Feb 3rd 2017, fumes and smoke in cockpit and cabin.