On 17 October 2023 a Boeing 737-800 passing FL130 after departing Manchester received a cabin pressurisation warning, and the crew recognised that both engine air bleeds were off. After correcting this, climb was continued without donning oxygen masks until an air conditioning pack fault occurred, which prompted the operator to request a return which was uneventful. The bleeds-off condition had arisen after maintenance and had then not been recognised at release to service or during crew preflight checks.