P0155 Code: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
The heater in bank 2's front O2 sensor, or its fuse, power feed or ECU-controlled ground, has failed.
At a glance
Severity: Low
- Meaning
- O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $1–$600
- DIY difficulty
- Moderate
The upstream sensor on bank 2 has a small electric heating element that brings its tip up to working temperature, well over 600 °F, within a minute or so of a cold start instead of waiting for the exhaust to do it. On most engines the heater gets 12 V through a fuse or relay while the ECU switches its ground side, and the ECU watches the resulting current or voltage. If that current is missing, too high or too low, it stores P0155. The key question is whether bank 2's heater is failing alone or along with others: on many V engines all four heaters share one fuse, so P0155 together with P0135, P0141 or P0161 points to the common supply, while a lone P0155 almost always means a burned-out element in the bank 2 sensor or damaged wiring on its way past the hot manifold.
Can I keep driving? Safe to drive. Bank 2 stays on its default mixture for longer after each cold start, so short trips cost a little more fuel, and the car won't pass an emissions test until it's fixed.
How to read P0155
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 55 Specific fault
Symptoms
- Check engine light, usually within the first few minutes after a cold start
- Bank 2 takes noticeably longer than bank 1 to enter closed loop
- Slightly higher fuel use on short trips and in winter
- Sometimes a companion P0154 because the unheated sensor stays silent
- Melted or discoloured sensor connector on the bank 2 side
- Otherwise no change in how the engine drives
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Heater element inside the bank 2 upstream sensor burned open
- Heater wires or connector melted by the bank 2 manifold or heat shield
- Blown shared O2 heater fuse or failed relay (other heater codes usually present)
- Corroded or loose heater pins in the connector
- Universal sensor spliced in with poor joints
- ECU heater driver fault
Diagnosis step by step
See which heater codes are stored
Read every stored and pending code. P0155 together with P0135, P0141 or P0161 means the shared feed is suspect: check the O2 heater fuse and relay first. P0155 on its own sends you straight to the bank 2 sensor and its wiring.
Tools: OBD-II scanner
Compare heater resistance with bank 1
On a cold engine unplug both upstream sensors and measure across the two heater pins on the sensor side; they are often the two wires of the same colour. Most narrowband heaters read somewhere around 3 to 15 ohms and wideband heaters about 2 to 4 ohms, but bank 1 gives you the right figure for your engine on the spot. An open circuit or a value far from bank 1's means a failed bank 2 heater.
Tools: Multimeter
Check the power feed at the harness
Key on, engine off: one heater pin in the harness connector should show battery voltage, roughly 12 to 12.6 V, against ground. No voltage there while the bank 1 connector has it means an open in the bank 2 branch of the feed, often right at the connector.
Tools: Multimeter
Confirm the ECU is switching the ground
With the sensor plugged in and the engine idling cold, back-probe both heater wires. The ECU pulses the ground side on and off, so the reading should be well above zero and may fluctuate. A steady 0 V difference means the ECU isn't switching the heater or the control wire is open.
Tools: Multimeter
Inspect the harness near the hot side
On V8 trucks the bank 2 harness runs over the manifold heat shield; on transverse V6 engines where bank 2 is the rear row it sits between the manifold and firewall and bakes. Look for a melted connector body, heat-hardened insulation and missing clips that let the wires drop onto hot metal.
Tools: Flashlight
Remove the old sensor without damaging threads
Rear-bank sensors often require removing the cowl panel, an engine cover or a heat shield, or working from below with a swivel O2 socket. Soak the threads, work on a warm engine, and if the threads are damaged, clean them with an M18 x 1.5 thread chaser before fitting the new sensor at about 30 lb-ft.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace blown O2 heater fuse (after finding the short) | Yes | $1–$15 |
| Replace bank 2 upstream O2 sensor yourself (part only) | Yes | $60–$220 |
| Replace bank 2 upstream O2 sensor at a shop, easy access | No | $180–$400 |
| Replace bank 2 upstream O2 sensor at a shop, rear bank at firewall | No | $280–$600 |
| Repair melted heater wiring or connector | No | $90–$280 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Toyota
On Toyota and Lexus transverse V6 engines bank 1 is the rear row against the firewall, so bank 2 is the more accessible front bank. Many of these use an air-fuel sensor upstream, and its heater circuit can report under other codes as well, so check the exact definition shown by your scan tool.
Emissions / smog test impact
Will fail
A check engine light for P0155 fails an OBD-II emissions or smog test. After the repair, a few cold starts are needed before the heater and O2 sensor monitors for bank 2 show complete.
Frequently asked questions
Can I fix just the heater?
No. The heater is sealed inside the sensor body, so a failed element means a new sensor. Do check the fuse, feed and ground first so you don't replace a good one.
Both P0135 and P0155 are on. Are both sensors bad?
Possibly, but check what they share first. On many engines the upstream heaters of both banks run off the same fuse or relay, and a single blown fuse sets both codes.
Why is the bank 2 sensor so much harder to replace?
On many transverse V6 engines and some V8s the bank 2 sensor sits at the back of the engine, close to the firewall, under heat shields or the intake. The part costs the same as bank 1's, but the labour can be double.
Does a bad heater hurt the engine?
Not directly. It mainly means bank 2 relies on default fueling for longer after each cold start. The practical effects are slightly higher fuel use and higher cold-start emissions.