P0137 Code: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

🟢 Low severity · drivable

Safe to drive — this downstream sensor only monitors the catalytic converter.

What P0137 means

The bank 1 downstream oxygen sensor is reporting low voltage. Because this sensor sits after the catalytic converter and only checks converter efficiency, low voltage here usually means the sensor or its wiring is at fault — or an exhaust leak is letting extra air in — rather than a fuel-mixture problem.

How the system works

Bank 1 sensor 2 is the oxygen sensor after the catalytic converter. With a healthy converter its signal is steady, typically around the middle of its range, because the converter has used up most of the spare oxygen. P0137 means the signal is staying low, which the computer reads as too much oxygen at that point in the exhaust.

That can be real extra oxygen, most often from an exhaust leak drawing in outside air near the sensor, or a false reading from a failed sensor or a wire shorted to ground.

Symptoms

Check engine light on
Usually no driving symptoms

Common causes

most common: Faulty downstream oxygen sensor
common: Exhaust leak near the sensor
less common: Shorted or damaged sensor wiring

How to diagnose P0137

Check for other codes. If lean codes such as P0171 or an upstream sensor code are also stored, deal with those first. A truly lean engine pulls the downstream reading low as well.
Watch the live reading. On a scan tool, a healthy downstream sensor holds fairly steady once warm. A reading stuck near zero that never moves points at the sensor or its wiring.
Look and listen for exhaust leaks. Check the joints and flex pipe around and ahead of the sensor for soot marks or a ticking noise on a cold start. Even a small leak there lets air in and drags the reading down.
Test whether the sensor responds. Briefly raise engine speed and hold it. The reading should rise from its low value. If it stays flat, the sensor is not responding.
Inspect the wiring. Check the harness for damage and for a signal wire shorted to ground, particularly where it runs close to the exhaust.

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor	Yes	$60–200
Repair an exhaust leak near the sensor	Mechanic	$50–200

Common misdiagnoses

Replacing the sensor while an exhaust leak is still present. The new sensor will report the same low reading.
Chasing fuel-mixture faults from this code alone. The downstream sensor does not control fuelling on most vehicles.
Replacing the catalytic converter. Converter efficiency is a different code (P0420).

P0137 vs similar codes

P0138: P0138 is the opposite reading on the same sensor: voltage stuck high, usually a failed sensor or a short to voltage.
P0136: P0136 is a general circuit fault on the same sensor, without saying whether the reading is high or low.
P0131: P0131 is a low reading on the upstream sensor, which is much more likely to reflect a real lean mixture.

Frequently asked questions

Is P0137 the same as P0131?

No. P0131 is the upstream sensor (fuel control); P0137 is the downstream sensor (catalyst monitoring). Low voltage on the downstream sensor rarely points to a genuine lean condition.

What causes P0137?

Usually a faulty downstream sensor or an exhaust leak before it.

How much does it cost to fix P0137?

Typically $60–$200 for a downstream oxygen sensor.

Does P0137 mean my catalytic converter is bad?

No. It describes the reading from the sensor after the converter. A failing converter is normally reported as P0420.

Can an exhaust leak cause P0137?

Yes. A leak near the sensor draws outside air into the exhaust between pulses, and the sensor reads that extra oxygen as low voltage.