P0137 on a Mazda: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — this downstream sensor only monitors the catalytic converter, so a Mazda with P0137 can be driven while you arrange the repair.

What P0137 means on a Mazda

On a Mazda, P0137 means the engine computer sees low voltage from the bank 1 downstream oxygen sensor, the one after the catalytic converter. This sensor only checks converter efficiency, so low voltage is usually a faulty sensor, damaged wiring, or an exhaust leak rather than a fuel problem. Four-cylinder Mazdas like the Mazda3 and CX-5 have a single bank.

Is P0137 common on Mazdas?

P0137 appears most often on higher-mileage Mazdas as the downstream sensor wears out. Older Mazda3 and Mazda6 models are common candidates, and small exhaust leaks before the sensor can produce the same low-voltage reading.

Commonly reported on: Mazda3, Mazda6, CX-5, CX-9.

Symptoms

Check engine light on
Usually no change in how it drives
Possible exhaust tick from a small leak
Slightly worse fuel economy if a leak is present

Common causes

most common: Faulty bank 1 downstream oxygen sensor
common: Exhaust leak before the downstream sensor
common: Sensor wiring damaged by exhaust heat
less common: Corroded sensor connector

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor	Yes	$60–220
Repair an exhaust leak near the sensor	Mechanic	$70–280
Repair damaged O2 sensor wiring	Mechanic	$60–200