Heater blowing lukewarm air — not hot like it used to

On the Cruze 1.4T, the #1 cause of lukewarm heat is a failed thermostat stuck partially open. The engine reaches operating temp (gauge reads normal) but the coolant isn't getting hot enough for good heater output. The Cruze thermostat is in the water outlet housing on top of the engine — super easy to replace, about  for the part and 30 minutes of work. There's also a Cruze-specific issue where the water outlet housing itself cracks. Inspect it while you're in there.

Second possibility: clogged heater core. Feel both heater hoses going through the firewall — both should be hot. If one is hot and one is lukewarm, the heater core has sediment blocking flow. You can try a heater core flush (disconnect both hoses, run a garden hose through it) before replacing it. Heater core replacement on a Cruze is an 8-hour dashboard-out nightmare, so try the flush first.