P0155 Code: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

What the P0155 code means

An oxygen sensor needs to get above roughly 300 degrees Celsius (about 570 °F) before it gives a useful signal, so it has a built-in heater element that the computer powers and monitors.

The P0155 code points to that circuit on the sensor ahead of the catalytic converter on bank 2, which is the side of the engine that does not have cylinder number one. Its bank 1 equivalent is P0135.

Because the engine has two banks, the diagnosis has an advantage you do not get on inline engines: there is a healthy twin sensor working on the other side. Its resistance, its current and its warm-up time give you an immediate reference, with no need to look up specifications.

The practical effect is the usual one for any heater failure. The engine starts and runs normally, but that bank takes longer to enter closed loop, fuel use goes up on short trips, and the related monitor never completes, so the vehicle will not pass an emissions test.

It helps to remember how the banks are numbered, because mixing them up is the most common mistake at the start. Bank 1 is always the one that contains cylinder number one, and that cylinder is not necessarily on the side you would expect. It depends on the numbering order the manufacturer picked for that specific engine. The vehicle documentation tells you without ambiguity, and looking it up costs less than taking apart the wrong side.

One access detail decides what this repair really costs. On many transverse V engines, bank 2 sits against the firewall, and its upstream sensor can require lifting the vehicle, removing shields or even shifting the engine on its mounts.

That changes the math: the part costs the same as on bank 1, but the labor can triple. Know this before you accept an estimate, and if the vehicle has high mileage, consider replacing the sensor on the other bank at the same time while the work is being done.

P0155 code symptoms

• Check engine light on.
• Higher fuel consumption on short trips than on the highway.
• The affected bank takes several minutes to enter closed loop.
• Slightly rough idle during the first few minutes.
• The oxygen sensor monitor never reaches ready status.
• Failed emissions test.
• Possible fuel mixture codes on the same bank.
• An estimate well above the one for the equivalent bank 1 sensor because of difficult access.

Likely causes of the P0155 code

1. Heater resistance open from age. The most frequent cause.
2. Blown fuse in the heater circuit.
3. Corroded or moisture-damaged connector.
4. Wiring damaged by exhaust heat.
5. Universal sensor with poorly made splices.
6. Power relay with worn contacts.
7. Poor ground in the circuit.
8. Sensor contaminated by oil consumption.
9. Failed output stage in the computer. The last thing to suspect.

How to diagnose the P0155 code step by step

1. Physically identify bank 2 before you take anything apart.
2. Compare with the bank 1 sensor. Measure the resistance of both heaters. If the healthy bank reads correctly and the suspect one does not, you have your confirmation right away.
3. Check for heater codes on other sensors. That would point to the fuse or to a shared power feed.
4. Check the circuit fuse before you take anything apart.
5. Verify 12 volts at the supply with the key on.
6. Measure the actual current with an amp clamp. A correct resistance with a weak supply sets the same code.
7. Inspect the connector and harness. They are heavily exposed to heat.
8. Check the circuit ground and its voltage drop.
9. Check how accessible the sensor is before you quote the job. On transverse V engines, bank 2 can require lifting the vehicle or shifting the engine.

How to fix the P0155 code

• Replace the oxygen sensor when the heater resistance is open.
• Replace the fuse and find out why it blew.
• Clean or replace the corroded connector and protect it with dielectric grease.
• Repair damaged wiring and cover it with heat-resistant sleeving.
• Replace the relay if its contacts are worn.
• Avoid universal sensors that need splices, because the added resistance can bring the P0155 code back.
• Restore the circuit ground down to bare metal.
• Apply anti-seize on the threads when you install the sensor, and work with the exhaust warm (not hot) so the sensor does not snap off in the pipe.
• Consider replacing the bank 1 sensor too if the vehicle has high mileage and the access is already open.

Technical reference data

• Typical heater resistance when cold: 2 to 14 ohms depending on the manufacturer.
• Current in the first seconds: 0.8 to 2 amps.
• Time to reach operating temperature: 20 to 30 seconds.
• Bank 2 is the side of the engine that does not contain cylinder number one.
• P0135 is the equivalent code on bank 1.
• Typical torque: 40 to 50 N·m (about 30 to 37 lb-ft).
• On transverse V engines, bank 2 usually sits against the firewall, with much harder access.

Brand-specific notes

The big advantage of diagnosing this code on a V engine is that you have a healthy twin on the other side. Measure both heaters and compare. It is faster and more reliable than hunting for the exact manufacturer specification.

If heater codes show up on both banks at the same time, stop suspecting the sensors. Two heater resistances failing at once is very unlikely. The fuse or the shared power feed is the likely cause.

Frequently asked questions about P0155