P0156🌫 Emissions / downstream oxygen sensorUsually OK to drive

P0156: O2 Sensor Circuit Malfunction (Bank 2, Sensor 2) (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 2 cited source(s)

What P0156 means

Sensor 2 is the downstream oxygen sensor, mounted after the catalytic converter on bank 2 (the side of the engine without cylinder 1; only V6, V8 and other engines with two banks have one). RepairPal explains that this rear sensor is used to judge how well the catalytic converter is cleaning the exhaust. OBD-Codes.com describes it as a four-wire sensor: the PCM supplies a ground and a reference voltage of about 0.5 volts, and the sensor varies that between about 0.1 volts (lean) and 0.9 volts (rich), with a separate battery feed and ground for its heater. The sources describe the trigger a little differently. OBD-Codes.com says P0156 sets when the sensor is stuck low for too long or isn't active at all, while RepairPal says it sets when the rear sensor signal does not show enough activity to indicate fuel control and closed-loop fuel management. Both point to the same sensor and circuit, and exact criteria vary by manufacturer.

Severity: can I keep driving?

Usually OK to drive

Get it checked within a reasonable time. RepairPal rates P0156 'Moderate', meaning it isn't usually an immediate safety risk, and both RepairPal and OBD-Codes.com say there are often no symptoms besides the check engine light. Left alone, it can lower fuel economy and, per RepairPal, cause an emissions test failure. If the check engine light is flashing, the engine runs very rough, or you smell fuel or see heavy black smoke, stop driving and have it looked at, because those point to a bigger problem than this sensor. Exhaust parts get very hot, so let the car cool fully before touching the oxygen sensor.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Faulty bank 2 sensor 2 oxygen sensor (listed first by both sources)
  2. Faulty sensor wiring or connections, including wiring damaged by contact with exhaust parts
  3. Signal circuit shorted to voltage (listed by OBD-Codes.com)
  4. Holes or leaks in the exhaust near the sensor, which can cause a false lean reading
  5. Water in the connector (named by OBD-Codes.com as something to check)
  6. Faulty PCM (only after the sensor and wiring check out, per OBD-Codes.com)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Read all stored codes. If misfire or fuel trim codes are also present, deal with those first.
  2. Make sure you have the right sensor: on bank 2, the side without cylinder 1, after the catalytic converter.
  3. With the exhaust cold, check the sensor wiring and connector for melted, chafed or broken wires, corrosion, or water in the connector, and make sure it is fully seated.
  4. Look and listen for exhaust leaks or holes near the sensor, such as a ticking or puffing sound at a cold start, or soot around joints.
  5. If you have a scan tool with live data, warm the engine and watch the bank 2 sensor 2 voltage. OBD-Codes.com says if it is stuck low but starts working when you raise the rpm for a few seconds, check for exhaust holes near the sensor; if the exhaust is intact, the sensor is sluggish and should be replaced.
  6. Clear the code and drive normally. If it comes back, the sensor or its circuit needs testing (see below).

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • OBD-Codes.com describes a test: if the voltage stays low even at higher rpm, unplug the sensor and watch the reading. If it rises to about 0.5 volts, check the connector and then replace the shorted sensor. If it stays low, suspect wiring. With the sensor unplugged, the signal wire on the PCM side should read about 0.5 volts and not above 1 volt; a higher reading means a short to voltage that needs repair. If that checks out, suspect the PCM.
  • RepairPal lists two common misdiagnoses: replacing the sensor when the real fault is wiring or connections, and replacing the rear sensor when the problem is an exhaust leak ahead of it. A shop can rule both out before buying parts.
  • The code returns after a new sensor, several oxygen sensor codes appear at once, or other heater or fuel codes are also present

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0156?

Usually, for a while. RepairPal rates it 'Moderate', and both sources say there may be no symptoms besides the check engine light. Get it diagnosed within a reasonable time, because it can lower fuel economy and cause an emissions test failure.

Which oxygen sensor is bank 2 sensor 2?

It is the sensor after the catalytic converter on bank 2, the side of the engine that does not have cylinder 1. Four-cylinder engines with a single bank don't have a bank 2 sensor.

What's the difference between P0156 and P0136?

They describe the same fault on different sides of the engine. P0136 is the downstream sensor on bank 1, and P0156 is the downstream sensor on bank 2. If both set together, a shared wiring or exhaust problem is worth checking before replacing both sensors.

Will replacing the oxygen sensor fix P0156?

Often, since both sources list a bad sensor first, but RepairPal warns that sensors are sometimes replaced when the real problem is wiring, connections, or an exhaust leak before the sensor. Check those first.

Sources

  1. OBD-Codes.com: P0156 O2 Sensor Circuit Malfunction (Bank 2, Sensor 2), by Dale Toalston
  2. RepairPal: P0156 OBD-II code (rated Moderate)

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.