P0138 on a Hyundai: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — this downstream sensor only monitors the catalytic converter, so a Hyundai with P0138 can be driven while you arrange the repair.

What P0138 means on a Hyundai

On a Hyundai, P0138 means the engine computer sees high voltage from the bank 1 downstream oxygen sensor, the one after the catalytic converter. This sensor only checks converter efficiency, so high voltage is almost always a faulty sensor or a wiring short rather than a rich-running engine.

Is P0138 common on Hyundais?

P0138 is most common on higher-mileage Hyundais as the downstream sensor fails. Sonata and Santa Fe models see it regularly, and wiring in the exhaust area can short after years of heat exposure.

Commonly reported on: Elantra, Sonata, Tucson, Santa Fe.

Symptoms

Check engine light on
Usually no change in how it drives
Possible reduced fuel economy
Sometimes no noticeable driving symptoms

Common causes

most common: Faulty bank 1 downstream oxygen sensor
common: Sensor wiring shorted to power
common: Exhaust leak before the sensor
less common: Failing catalytic converter

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor	Yes	$60–230
Repair shorted sensor wiring	Mechanic	$70–250
Replace the catalytic converter if it's failing	Mechanic	$800–2500