Subaru Outback · Diagnostic Trouble Code
P0155 — O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
What it means, how urgent it is, the most likely causes, and the exact order to diagnose it — with the repair guides that fix it.
What P0155 means
The internal heater brings the sensor to operating temperature within seconds of startup so fuel control goes closed-loop quickly. The PCM monitors the heater's current draw and sets P0155 when the circuit is open, shorted, or drawing abnormal current.
On V6 and V8 engines, Bank 2 is the side opposite cylinder 1. When P0155 appears together with P0135 (Bank 1's heater code), look at what the two banks share — usually a fuse, relay, or power feed — before buying two sensors.
⚠️ How urgent is it?
Repair before your emissions test. If P0135 is present too, check the shared heater fuse and relay first — one blown fuse explains both codes.
Most likely causes
- A burned-out heater element in the Bank 2 upstream oxygen sensor — the most common cause.
- Heater circuit wiring faults: blown fuse, damaged wires, corroded connector.
- Heat-related wiring degradation near the Bank 2 exhaust manifold.
- Rarely, a PCM heater-driver issue.
How to diagnose it — in order
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1
Check the heater fuse — shared fuses can take out both banks' heaters at once.
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2
Measure heater resistance at the Bank 2 Sensor 1 connector with the sensor unplugged; open circuit condemns the sensor.
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3
Confirm battery voltage at the heater feed wire, key on.
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4
Trace the Bank 2 harness for melted or chafed sections near the manifold.
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5
If both P0135 and P0155 set together, suspect the shared fuse, relay, or power feed before replacing two sensors.
Reading the code is step zero
A basic OBD2 scanner plugs into the port under your dash and tells you exactly which codes are stored — plus freeze-frame data showing what the engine was doing when the code set.
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Repair guides that fix P0155
General diagnostic information, not a factory service procedure. If you're unsure, have a qualified technician confirm the diagnosis before buying parts.