P0155 is a generic OBD-II diagnostic trouble code that means the Engine Control Module (ECM/PCM) has detected a malfunction in the oxygen sensor heater circuit for Bank 2 Sensor 1. Bank 2 is the engine bank that does not contain cylinder #1, while Sensor 1 is the upstream oxygen sensor located before the catalytic converter.
The oxygen sensor heater allows the sensor to reach its operating temperature quickly so the engine control system can begin using accurate oxygen-sensor information sooner. A P0155 fault can be caused by a failed oxygen sensor heater, an open or shorted circuit, a blown fuse, damaged wiring, poor electrical connections, low supply voltage, or a control-side problem.
Code: P0155
Description: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
Code Type: Generic OBD-II
System: Fuel and Emission Control
Sensor Location: Bank 2, Sensor 1 — Upstream
Severity: Moderate
P0155 Code Meaning
The P0155 code means the PCM has detected an abnormal electrical condition in the heater circuit of the Bank 2 Sensor 1 heated oxygen sensor (HO2S). Depending on the vehicle, the PCM may detect an open circuit, excessive resistance, excessive current, a short circuit, or another electrical condition outside its calibrated range.
The fault concerns the heater circuit, not necessarily the oxygen-sensor signal circuit itself. Therefore, replacing the sensor without testing the heater power, ground/control circuit, fuse, connector, and wiring can result in an unnecessary repair.
What Are Bank 2 and Sensor 1?
Bank 2 identifies the engine cylinder bank that does not contain cylinder #1. On many V6, V8, and other engines with two cylinder banks, there is a Bank 1 and Bank 2.
Sensor 1 refers to the upstream oxygen sensor located before the catalytic converter. Therefore, P0155 specifically concerns the upstream O2 sensor on Bank 2.
Sensor locations can vary between manufacturers and engine designs. Always verify the exact sensor location using the vehicle's service information.
P0155 Symptoms
- Check Engine Light illuminated.
- Longer time before the oxygen sensor reaches operating temperature.
- Reduced fuel economy in some vehicles.
- Higher emissions.
- Engine may remain in open loop longer than intended.
- Possible rough operation during cold operation.
- Additional oxygen-sensor or fuel-mixture codes may be stored.
Common Causes of P0155
- Failed Bank 2 Sensor 1 oxygen-sensor heating element.
- Open heater power circuit.
- Open heater ground or control circuit.
- Short circuit in the heater wiring.
- Damaged or melted wiring near the exhaust.
- Corroded, loose, or water-damaged connector.
- Blown oxygen-sensor heater fuse.
- Low battery or charging-system voltage.
- Incorrectly installed replacement sensor.
- Incorrect sensor connector or aftermarket wiring.
- PCM heater-control circuit fault.
P0155 Severity
Severity: Moderate.
P0155 does not normally indicate an immediate mechanical engine failure, but it should not be ignored. A malfunctioning heater can delay oxygen-sensor operation and increase emissions. Depending on the vehicle, fuel economy and cold-start operation may also be affected.
Can You Drive With P0155?
In many cases the vehicle can still be driven with P0155, particularly if the engine runs normally and the Check Engine Light is steady. However, prolonged driving is not recommended because the vehicle may have increased emissions and reduced fuel-control efficiency.
P0155 Heater Circuit Operation
The heated oxygen sensor contains a small electrical heating element. The heater brings the sensor up to operating temperature much faster than exhaust heat alone.
The exact circuit design varies by vehicle. Some systems provide battery voltage to the heater and use the PCM to control the ground side. Other designs may use different control strategies.
Do not assume wire colors, pin numbers, heater resistance specifications, or control polarity are universal. Always use the wiring diagram for the specific vehicle.
P0155 Typical Wiring Diagram
P0155 Sensor Pinout
Many heated oxygen sensors use four wires: two for the oxygen-sensor signal/reference circuit and two for the heater. However, connector layouts and wire colors vary considerably between manufacturers.
P0155 Diagnostic Procedure
Step 1 — Scan All Stored Codes
Connect an OBD-II scan tool and record all stored, pending, and history codes. If P0155 appears together with other heater-circuit codes, the common power supply, fuse, relay, or wiring should be investigated first.
Step 2 — Confirm Bank 2 Sensor 1
Identify Bank 2 and locate Sensor 1 using the vehicle-specific service information. Do not assume the left or right side of the engine is always Bank 2 because engine layouts differ.
Step 3 — Inspect the Sensor and Harness
- Inspect the connector for corrosion.
- Check for loose or pushed-back terminals.
- Look for broken wires.
- Inspect for melted insulation.
- Check whether the harness is touching the exhaust.
- Look for water or oil contamination.
Step 4 — Check the Heater Fuse
Locate the fuse supplying the oxygen-sensor heater circuit. Test the fuse rather than relying only on visual inspection. If the fuse is blown, determine why it failed before installing another fuse.
Step 5 — Check Heater Power
With the sensor disconnected and the ignition in the appropriate test position, measure voltage at the heater supply terminal according to the manufacturer's procedure.
A missing supply voltage can indicate a blown fuse, relay problem, open circuit, wiring fault, or power-distribution problem.
Step 6 — Measure Heater Resistance
Disconnect the oxygen sensor and measure resistance across the heater terminals using a digital multimeter. Compare the result with the manufacturer's specification.
An open circuit or resistance substantially outside the specified range can indicate a defective heater element.
Step 7 — Check the Control Circuit
If power and sensor resistance are correct, inspect the heater control/ground circuit between the sensor and PCM. Check for an open circuit, short to power, short to ground, excessive resistance, or poor terminal contact.
Step 8 — Clear the Code and Perform a Drive Cycle
After repairing the confirmed fault, clear the DTC and perform the manufacturer's recommended drive cycle. Recheck the system for P0155 and verify that the oxygen-sensor heater monitor completes successfully.
P0155 Diagnostic Flowchart
P0155 Heater Circuit Electrical Tests
The following measurements are diagnostic concepts rather than universal specifications. Exact voltage, resistance, duty cycle, and test conditions must come from the vehicle manufacturer's service information.
Power present + heater resistance incorrect → suspect sensor heater.
Power missing → trace fuse, relay, power supply, and wiring.
Power and resistance correct + control fault → inspect control wiring and PCM driver.
Intermittent readings → inspect connector tension, corrosion, vibration damage, and harness routing.
P0155 Oscilloscope Testing
An oscilloscope can help diagnose intermittent heater-control problems that may not be visible with a simple resistance test. The exact waveform depends on the vehicle's heater-control strategy.
A missing control signal, abnormal duty cycle, unstable waveform, or excessive voltage drop can help identify wiring or PCM-control problems. Always use the manufacturer's test procedure when interpreting the waveform.
P0155 Live Data
- Engine coolant temperature.
- O2 sensor status.
- Closed-loop/open-loop status.
- O2 sensor voltage or equivalence-ratio data where available.
- Heater current or heater duty cycle on vehicles that report it.
- Fuel trims.
- Battery/charging voltage.
If the vehicle supports heater current or heater duty-cycle PID data, compare the actual value with manufacturer specifications rather than relying on a generic expected number.
Common Repairs for P0155
- Replace the defective Bank 2 Sensor 1 oxygen sensor.
- Repair an open or damaged heater wire.
- Repair a shorted heater circuit.
- Replace a blown fuse after correcting the cause of the failure.
- Repair corroded or damaged connector terminals.
- Repair damaged harness routing near the exhaust.
- Correct low system voltage or charging problems.
- Repair the PCM control circuit when confirmed by testing.
Common P0155 Diagnostic Mistakes
- Replacing the sensor without testing heater resistance.
- Ignoring a blown fuse.
- Assuming all O2 sensor wire colors are identical.
- Using a generic pinout instead of the vehicle wiring diagram.
- Failing to inspect wiring close to the exhaust.
- Testing resistance while the sensor is still connected to the circuit.
- Ignoring low battery or charging voltage.
- Replacing the PCM before testing the complete circuit.
- Using an incorrect or poor-quality replacement sensor.
P0155 Repair Cost
Repair cost varies significantly by vehicle, sensor accessibility, parts quality, labor rates, and the actual cause of the fault.
Fuse or connector repair: usually low cost.
Wiring repair: low to moderate depending on access.
O2 sensor replacement: moderate and vehicle-dependent.
PCM or major harness repair: potentially expensive.
Recommended Diagnostic Tools for P0155
- OBD-II diagnostic scanner.
- Digital multimeter (DMM).
- Automotive test light when appropriate.
- Oscilloscope for advanced diagnosis.
- Back-probing test leads.
- Vehicle-specific wiring diagrams.
- Manufacturer service information.
P0155 OEM Notes
The exact diagnostic strategy for P0155 varies between manufacturers. Some vehicles monitor heater current directly, while others monitor voltage, resistance, or the electrical response of the heater circuit.
Connector pin numbers, wire colors, fuse locations, heater resistance specifications, and PCM control strategy are therefore not universal.
For accurate diagnosis, use the service information for the exact year, make, model, engine, and VIN configuration.
P0155 Related OBD-II Codes
- P0135 — O2 Sensor Heater Circuit Malfunction, Bank 1 Sensor 1
- P0141 — O2 Sensor Heater Circuit Malfunction, Bank 1 Sensor 2
- P0161 — O2 Sensor Heater Circuit Malfunction, Bank 2 Sensor 2
- P0130 — O2 Sensor Circuit Malfunction, Bank 1 Sensor 1
- P0150 — O2 Sensor Circuit Malfunction, Bank 2 Sensor 1
- P0151 — O2 Sensor Circuit Low Voltage, Bank 2 Sensor 1
- P0152 — O2 Sensor Circuit High Voltage, Bank 2 Sensor 1
- P0153 — O2 Sensor Circuit Slow Response, Bank 2 Sensor 1
- P0154 — O2 Sensor Circuit No Activity Detected, Bank 2 Sensor 1
P0155 FAQ
What does P0155 mean?
P0155 means the PCM has detected a malfunction in the heater circuit of the Bank 2 Sensor 1 oxygen sensor.
Which O2 sensor is P0155?
P0155 refers to Bank 2 Sensor 1, the upstream oxygen sensor on the engine bank that does not contain cylinder #1.
Is P0155 a bad oxygen sensor?
It can be, but not necessarily. A blown fuse, open wire, short circuit, connector problem, low voltage, or PCM control fault can also produce P0155.
Can a blown fuse cause P0155?
Yes. If the oxygen-sensor heater receives its power through a fuse and that fuse is open, the heater circuit may fail and P0155 can be stored.
Can I drive with P0155?
Usually, a vehicle with a steady Check Engine Light and normal drivability can be driven for a limited period, but the fault should be diagnosed and repaired rather than ignored.
Does P0155 affect fuel economy?
It can. Because the heater helps the oxygen sensor reach operating temperature quickly, a heater fault can delay normal sensor operation and may affect emissions and fuel control depending on the vehicle strategy.
How do I fix P0155?
First identify the failed part of the circuit. Inspect the sensor and wiring, check the fuse and heater power supply, measure heater resistance, test the control circuit, and replace or repair only the component confirmed to be faulty.
Final Result
P0155 indicates an O2 Sensor Heater Circuit Malfunction on Bank 2 Sensor 1. The most effective repair begins with electrical diagnosis rather than immediately replacing the oxygen sensor.
Check the fuse, power supply, heater resistance, wiring, connector, control circuit, and system voltage. If the heater element is outside the manufacturer's specification, replacing the Bank 2 Sensor 1 oxygen sensor may be the correct repair. If the sensor tests correctly, continue diagnosing the vehicle wiring and PCM control circuit.