DTC Description
WARNING: This page is about a different car, the 2021 Hyundai Kona. However, it is still accessible from the selected car via links, so may be relevant.
ECM checks output signals from HO2S under detecting condition. ECM judges it's signals too slow and sets P013900, if output signal from rear HO2S isn't increased up to specification level within a short time after fuel cut.
(This DTC might be caused by the malfunction of Heater circuit, so, check the heater circuit first).