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P0155 Generic (SAE)

O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)

Updated 2026-10-02 · Fuel & air metering

Severity
Low
Typical repair cost
30 – 300 EUR
System
Powertrain

What it means

P0155 concerns only the heater circuit of the bank 2 upstream oxygen sensor, not the signal. The heater is a resistive element (typically 2-15 ohm) fed 12 V through a relay or fuse and switched by the ECU on the ground side; it brings the sensor to its 300-600°C operating temperature within 30-60 seconds of a cold start. The ECU detects an open, short, or wrong current draw. Bank 2 is the bank that does not contain cylinder 1.

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Symptoms

  • Check engine light, often appearing only after a cold start
  • Extended open-loop running after start – richer mixture and higher consumption for the first minutes
  • Slightly rough idle during warm-up
  • Possible companion code P0154 (no activity) if the element stays cold
  • Rarely a blown fuse that also disables other sensor heaters

Common causes

  1. Burned-out heater element inside the sensor (most common, especially past 150,000 km)
  2. Blown fuse or failed relay supplying 12 V to the sensor heaters
  3. Corroded or spread connector pins on the heater wires
  4. Heater wire chafed through on the exhaust and shorted to ground
  5. Open ground-side control wire to the ECU
  6. Faulty ECU heater driver transistor (uncommon but seen after a shorted sensor)

How to diagnose

  1. Unplug the sensor and measure resistance across the two heater pins (usually the two same-coloured wires, often white): 2-15 ohm is normal, with most sensors reading 5-8 ohm at 20°C; infinity means an open heater, under 1 ohm means a shorted heater
  2. Check the fuse for the O2 heaters (frequently shared with the injectors or ignition circuit) and the relay; a blown fuse often indicates a shorted heater that must still be replaced
  3. With the key on, measure voltage at the harness heater supply pin: it should read battery voltage (12-14 V); if absent, trace back to the fuse or relay
  4. Use a test light or scope on the ground-side control wire: the ECU should pull it low (0 V) with the engine running, often in a PWM pattern after warm-up
  5. Measure heater current with an ammeter in series: expect roughly 1-2 A at 12 V for an 8-ohm heater; a current far below this points to a high-resistance connection
  6. If the heater, supply and ground all check out, swap the bank 1 and bank 2 sensors and see whether the code follows the sensor

Frequently asked questions

Does a heater fault affect driving?

Only slightly. The exhaust heats the sensor on its own after a few minutes, so the ECU runs open-loop for longer after cold starts and the car uses a little more fuel; the main consequence is a failed inspection.

Why does the fuse keep blowing?

A heater element that has shorted internally draws far more than its rated 1-2 A. Replace the sensor, not just the fuse.

Can I use a universal sensor with spliced wires?

It works if soldered and heat-shrunk properly, but the heater wires carry 1-2 A and poorly crimped joints become a fresh P0155 within months. OEM-connector sensors are safer.

Related codes

P0150P0154P0135P0161P0141

Nearby codes in this group