3921004AA5 Hyundai / KIA SENSOR ASSY-OXYGEN,RR, Price: 136.16$, Weight: 0.107kg

Rear Oxygen Sensor for Hyundai & Kia (Downstream O2)

This rear oxygen sensor is a key emissions and fuel-control spare part used on many Hyundai and Kia engines. It sits after the catalytic converter and “sniffs” the exhaust to confirm the cat is doing its job. When you replace it on time, you keep the engine running clean, protect the catalytic converter, and help the car pass inspection ✅

Why this sensor is needed

The ECU needs a stable downstream oxygen signal to compare against the front sensor. That comparison helps the system fine-tune emissions strategy and detect catalyst efficiency problems. A healthy sensor also prevents unnecessary rich running that can waste fuel ⛽

How it works

Inside is a heated zirconia element. The built-in heater brings it up to operating temperature fast, then it generates a voltage based on oxygen content in the exhaust. The ECU reads that signal and adjusts monitoring and catalyst control logic

What happens if it’s broken

Common signs include a check engine light, failed emissions test, sluggish response, higher fuel use, and catalyst damage risk from incorrect mixture corrections. In some cases you may notice exhaust smell and unstable idle ⚠️

Technician tip

Use anti-seize on threads (if not pre-coated) and avoid twisting the harness during install for long sensor life.