Hyundai Theta II

Hyundai's all-aluminium four-cylinder family, shared across Hyundai and Kia and built in Korea and at Hyundai's Alabama plant. It is the volume petrol engine of the Sonata, Santa Fe, Optima and Sorento through the 2010s, in naturally aspirated 2.4 GDI and turbocharged 2.0 T-GDI forms.

This family is the subject of some of the largest engine recalls in the industry, and that is the first thing to establish about any example. The failure is connecting-rod bearing wear leading to seizure — often preceded by a knocking noise, sometimes by nothing at all, and in a proportion of cases ending in engine failure while driving. Manufacturing debris left in the crankshaft oil passages was identified as a cause on affected production.

The practical consequences are unusual and worth stating plainly. A knocking noise on a Theta II is not something to monitor. Recall and extended-warranty coverage has been broad but is keyed to specific engines and build ranges, so check the VIN against open campaigns before quoting any repair — the same fault can be a covered replacement on one car and a customer-paid one on the next. Many affected engines also received a knock-sensor detection software update, which changes what the car will do when the condition begins.