OBD-II Code P0155 — What It Means & How to Fix It

What Does This Error Mean?

P0155 means the built-in heater inside your Bank 2 upstream oxygen sensor has failed. Modern O2 sensors have a small heater that warms them up quickly after a cold start. This heater helps the sensor reach operating temperature in about 30 seconds instead of several minutes. When the heater fails, the sensor takes much longer to work — hurting cold-start fuel economy. This is usually a cheap fix.

Affected Models

All vehicles 1996+
Common in Toyota Camry and Highlander
Common in Lexus RX and ES
Common in Honda CR-V and Pilot
Common in Ford Explorer and Expedition

Common Causes

Burned-out heater element inside the oxygen sensor itself
Blown fuse in the heater circuit cutting power to the sensor heater
Broken or melted wire in the heater circuit from exhaust heat
Corroded or loose connector at the oxygen sensor plug
Faulty PCM relay not supplying power to the heater circuit

How to Fix It

Check the fuse box for any blown fuses related to the oxygen sensor or exhaust heater circuit. Look in both the under-hood fuse box and the interior fuse panel. Your owner's manual or the fuse box lid will identify which fuse to check.

A blown heater fuse is the easiest and cheapest fix — a pack of replacement fuses costs about $3.
Inspect the oxygen sensor connector and wiring harness on Bank 2. Oxygen sensors sit near hot exhaust pipes and wires can melt, crack, or chafe over time. Look for any visibly damaged insulation, melted plastic, or green corrosion at the connector.

Pull the connector apart and look at the individual pins — bent or corroded pins are a common cause and can be repaired without replacing the full wiring harness.
Use a multimeter to test the heater circuit. With the sensor unplugged and the ignition on, check for battery voltage (around 12V) at the heater power wire at the connector. If there's no voltage, the fault is in the wiring — not the sensor.

The oxygen sensor connector typically has 4 wires: two for the heater circuit and two for the signal circuit. Your service manual will identify which is which.
Replace the Bank 2 upstream oxygen sensor if voltage is present at the connector but the code persists. An internal heater element failure is the most common cause of P0155 when the wiring checks out fine.

Always use a quality replacement sensor. Cheap off-brand sensors sometimes have unreliable heater elements and can fail again quickly.
Clear the code and verify the fix by driving through at least two cold-start cycles. Start the car cold, warm it up to full operating temperature, and check for the code. A successful fix means the code stays gone through multiple cold starts.

P0155 is only tested during cold start warm-up — the ECM won't flag it if the car was already warm when you started driving.

When to Call a Professional

If replacing the sensor doesn't fix P0155, have a mechanic trace the heater circuit. They'll use a wiring diagram and multimeter to check fuses, relays, and the full wiring path. This is straightforward diagnostic work that costs $75-$125 at most shops. Avoid guessing on wiring repairs — a bad splice can create new problems.

Frequently Asked Questions

Is P0155 a serious problem?

P0155 is considered a minor code — it won't leave you stranded. However, a cold sensor means the engine runs in open-loop mode longer after startup. Open-loop mode uses more fuel and creates more emissions during the warm-up phase. It's worth fixing but not an emergency.

Will P0155 cause me to fail emissions testing?

Yes, P0155 can cause an emissions test failure even if your actual emissions are fine. Most states check OBD-II readiness monitors — the heater circuit monitor is one of them. If the heater monitor shows 'not ready' or 'fail,' the inspector will fail the vehicle. Fix the code and clear it, then drive enough cycles for all monitors to reset before retesting.

Can I replace just the heater element, or do I need a whole new sensor?

You cannot replace just the heater element — it's built into the sensor and not serviceable separately. You'll need to replace the entire oxygen sensor as one unit. Fortunately, oxygen sensors are relatively inexpensive at $25-$80 for most common vehicles.