P0155
O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)P0155 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0155 means
P0155 is set when the PCM detects a malfunction in the heater circuit of the upstream (pre-catalyst) oxygen sensor located on Bank 2 — the exhaust bank opposite cylinder #1. Modern wideband and narrowband lambda sensors embed a resistive heating element so the ceramic tip reaches operating temperature (~350 °C) within 20–30 seconds of cold start, enabling closed-loop fuel control before exhaust heat alone could warm the sensor.
The PCM controls heater current either by direct ground-side switching or by a dedicated relay, and it monitors heater circuit current or voltage drop. If current is absent, excessively high (shorted element), or the circuit is open — and this condition persists beyond the manufacturer's threshold — the code sets and the MIL illuminates. The sensing element itself may be fully functional; only the heater circuit has failed.
The practical consequence is a longer cold-start open-loop period during which the PCM runs on a fixed fuel map instead of sensor feedback. This increases cold-start emissions and fuel consumption but rarely causes severe driveability issues at operating temperature. P0155 will not clear until the heater circuit passes two successful warm-up monitor cycles after the repair.
Common causes
Most-frequently reported root causes when P0155 is logged.
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1
Failed (open or high-resistance) heater element inside the sensor — most common on sensors over 100,000 km
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2
Broken or corroded power supply wire to the heater circuit
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3
Corroded or damaged ground wire / ground splice for the heater circuit
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4
Blown fuse or failed relay supplying battery voltage to the heater
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5
Wiring harness chafed against exhaust heat shield creating an intermittent short
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6
Water intrusion into the sensor connector causing corrosion and high resistance
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7
PCM output driver failure (rare — only after ruling out all external wiring faults)
Symptoms drivers notice
How to diagnose P0155
A typical diagnostic flow when this code is present.
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1
Check for any blown fuse associated with the O2 sensor heater supply circuit in the underhood fuse box
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2
Disconnect the Bank 2 Sensor 1 connector and back-probe the heater power pin with a multimeter; expect near battery voltage (12–14 V) with ignition on
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3
Verify the heater ground pin shows 0–0.2 V to chassis ground with the circuit active
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4
Measure heater element resistance across the two heater pins (engine cold); a typical narrowband element reads 3–20 Ω — an open circuit (OL) confirms element failure
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5
Inspect the wiring harness from the sensor to the PCM / relay for heat damage, chafing, or moisture ingress
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6
If power, ground, and wiring all check good, perform a PCM pin-out resistance and voltage test before condemning the PCM
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7
Replace the oxygen sensor if the heater element is confirmed open or out of specification
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
Will P0155 cause poor engine performance at normal driving temperature?
Generally no. Once the sensor element reaches operating temperature from exhaust heat alone (typically 1–3 minutes after start), closed-loop control functions normally. The main impact is during cold starts when the heater is needed most.
Can I test the heater element without a scan tool?
Yes. Disconnect the sensor connector, set a multimeter to resistance (Ω), and probe the two heater pins. A reading between roughly 3 Ω and 20 Ω is healthy; an open circuit (OL / infinite resistance) means the element has burned out and the sensor must be replaced.
Is it safe to ignore P0155 and keep driving?
The vehicle is driveable, but extended cold-start open-loop operation increases fuel consumption and tailpipe emissions. On vehicles with strict emissions monitoring, it may also trigger a smog test failure. Replacing the sensor is inexpensive and should not be deferred long.
Does P0155 affect the downstream (post-cat) sensor on Bank 2?
No. P0155 only concerns Bank 2 Sensor 1 (upstream/pre-catalyst). The downstream sensor has its own heater circuit and separate fault codes (P0161 for heater malfunction on Bank 2 Sensor 2).
Disabling P0155 in software
RaceTune can permanently disable P0155 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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