Honda O2 Sensor: Symptoms, Codes, Location & Replacement

A failing oxygen sensor is one reason a Honda’s check engine light comes on. It can also set codes that look like a catalytic converter problem. This guide covers what the O2 sensor does, how a bad one shows up, which codes it sets, where the sensors sit, and how to replace one yourself.

What the oxygen sensor does

Your Honda’s oxygen (O2) sensors measure how much unburned oxygen is left in the exhaust and report it to the engine computer. The computer uses that reading to fine-tune the air-fuel mixture. There are usually two per exhaust line: an upstream sensor (before the catalytic converter) that controls fueling, and a downstream sensor (after the converter) that monitors how well the converter is working.

Symptoms of a bad Honda O2 sensor

• Check engine light with an oxygen-sensor or fuel-trim code.
• Poor fuel economy — a lazy sensor makes the engine run rich.
• Rough idle or slight hesitation.
• A failed emissions test.
• Sometimes a P0420 that is actually the downstream sensor, not the converter.

O2-sensor related codes

• P0135 / P0141 — O2 sensor heater circuit (upstream / downstream).
• P0136 / P0137 / P0138 — Downstream O2 sensor circuit issues.
• P0171 — System too lean; a faulty upstream sensor is one possible cause.
• P0420 — Catalyst efficiency below threshold, which a faulty downstream sensor can trigger.

Because a worn O2 sensor can set the same P0420 code as a failing converter, always confirm with a scan before assuming the worst. A basic OBD2 scanner shows you exactly which sensor and circuit is reporting the fault.

Where the O2 sensors are on a Honda

On most Hondas, the upstream sensor (bank 1, sensor 1) threads into the exhaust manifold or front pipe, and the downstream sensor (sensor 2) screws in after the catalytic converter. V6 models such as the Pilot and Odyssey have sensors on both banks. Knowing which sensor a code refers to keeps you from replacing the wrong one.

Replacing a Honda O2 sensor

An oxygen sensor can be a DIY job if you can reach it. You’ll need the correct replacement oxygen sensor for your model and an O2 sensor socket, which has a slot for the wiring. Let the exhaust cool, unplug the connector, unscrew the old sensor, thread in the new one, and clear the code. Stick with an OEM or OEM-quality part.

When the code points at the converter instead, see the Honda catalytic converter guide.