OBD-II

P0155 Diagnostic Guide (OBD-II)

What This Code Means

• Code: P0155
• General meaning.
• Open-source mapping note: An active GitHub/open-source definition states i.e., O2 sensor heater circuit malfunction for a sensor in a given bank. This confirms the general heater-circuit theme but bank/sensor numbering can vary by vehicle . In practice, P0155 maps to the heater circuit issue for a particular bank/sensor pair depending on the vehicle .

Bank/sensor mapping caveat

• The exact bank and sensor pairing for P0155 varies by vehicle make/model and the OBD-II protocol implementation. Some definitions refer to Bank 2 Sensor 2, while others may reference Bank 1 Sensor 2 or different upstream/downstream placements. The open-source definition indicates in one mapping, while many vehicle diagnostics follow Bank 2 Sensor 2 for P0155. For precise mapping, consult the vehicle's service information/diagnostic protocol or the OEM data for your specific year/model. If you're diagnosing a vehicle, confirm the bank/sensor placement with the factory wiring diagram.

Symptoms

• MIL illuminated or flashing (malfunction indicator lamp) with P0155 stored in the ECU.
• After a cold start, the MIL may stay on due to the heater not reaching operating temperature promptly.
• Possible minor or no drivability impact, but potential for increased emissions and longer trip times to complete emissions readiness.
• Potentially reduced catalyst efficiency until the O2 sensor heater reaches operating temperature.
  Note: The symptoms are consistent with a diagnostic trouble code category for O2 sensor heater circuit faults.

Probable Causes

• O2 sensor heater element failure (sensor itself): 40-60%
  • The heater inside the O2 sensor can fail open or develop a short, preventing the sensor from warming quickly. This is a common root cause for P0155-type codes.
• Wiring harness damage, poor/loose connector, or damaged insulation (sensor-to-PCM harness): 20-30%
  • Damaged wires, pin corrosion, or a poorly seated connector can interrupt the heater supply or ground path.
• Fuse(s) or power/ground supply issues to the heater circuit: 5-15%
  • A blown fuse or a degraded/failed circuit path can cut heater power or ground.
• PCM/ECU output control issue or ground fault: 0-5%
  • Infrequent, but possible; the PCM controls heater power in some designs. A misdrive or degraded ground reference can mimic a heater fault.
• External conditions or wiring contamination (heat damage, abrasion, moisture): 0-5%
  • Harsh environment in the exhaust area can accelerate harness damage or corrosion.
    Note: All percentages are approximate, reflecting general field patterns in absence of NHTSA complaint data. They are intended to guide prioritization during troubleshooting.

Diagnostic Approach

1) Confirm the code

• Use a scan tool to confirm P0155 is present and note any related codes (e.g., P0150-P0154 family, or codes for nearby sensors). Check freeze-frame data and readiness monitors. Document any additional DTCs to understand system interactions.

2) Visual inspection and basic electrical checks

• Inspect the O2 sensor and its wiring/connector for obvious damage, corrosion, heat wear, or moisture intrusion. Check for proper connector engagement.
• Inspect the wiring harness near the sensor for abrasion, chafing, or pin damage.
• Check visible fuses and relays related to the O2 sensor heater circuit (fuse rating varies by vehicle; consult the vehicle's fuse diagram in the owner/service manual).
• Safety note: Disconnect power before unplugging or probing if required by the service procedure.

3) Circuit-level checks (with engine off and key off, then with engine on as appropriate)

• Check supply voltage to the heater circuit (12V) and the ground path:
  • With a multimeter, verify that the heater circuit receives proper voltage when the PCM commands the heater ON (if the vehicle scan tool indicates heater ON). Also verify a solid ground path to the sensor.
• Measure the heater resistance of the O2 sensor (the heater element resistance varies by sensor type; consult service data for the exact value). A resistance that is far from specification or shows a short/open indicates a fault in the sensor heater element.
• If accessible, check the heater circuit for continuity from the sensor to the PCM/ground using the service manual's wiring diagram.

4) Functional testing and swap/verification steps

• If a known-good spare sensor is available or you have a bank with two sensors, temporarily swapping the suspect sensor with a known-good sensor can help determine if the issue is sensor-based or wiring/ECU related.
• After any sensor swap or wiring repair, clear the codes and perform a test drive to confirm whether P0155 recurs and whether the heater is reaching operating temperature (observe O2 sensor heater status via the scan tool if possible).

5) Repair options by scenario

• If the heater circuit is open/shorted or the sensor heater itself is faulty:
  • Replace the O2 sensor (Bank/sensor mapping depends on vehicle; ensure replacement sensor is suitable for upstream/downstream and bank as indicated by your bank/sensor mapping for the vehicle).
• If wiring/connectors are damaged:
  • Repair or replace damaged wiring; reseat and reseal connectors; repair any damaged insulation; ensure proper grounding and shield integrity.
• If a fuse/relay is defective:
  • Replace the fuse/relay and re-test the circuit.
• If PCM/ECU control is suspected:
  • This is less common; verify with OEM service data and consider ECM reflash or replacement only after ruling out harness and sensor faults.

6) Recheck and validation

• Clear codes and perform a road test to ensure P0155 does not return.
• Confirm that the O2 sensor heater is warming up and reaching operating temperature quicker after starting the engine.
• Confirm no other O2 sensor codes are present unless they're related to a broader O2 system issue.
• Ensure the vehicle passes emission testing readiness checks if applicable (Emissions Testing context: OBD-II readiness monitors).

Observations about related literature

• OBD-II literature explains that diagnostic trouble codes are used by the system to monitor various parameters and flag issues, including heater circuits in O2 sensors. This supports the focus on the heater circuit as the root cause class for P0155.
• Emissions testing sections emphasize the role of OBD-II monitors in emissions readiness; heater circuit faults can affect sensor operation and readiness status. This is relevant when planning emissions-related repairs or if the vehicle is undergoing testing.

Notes

• Bank/sensor mapping: As noted, the exact sensor/bank pairing for P0155 can vary. Do not rely on a single mapping for all vehicles. Always confirm with the vehicle's service information or factory wiring diagrams.
• Diagnostic scope: P0155 may interact with other O2 sensor codes; addressing a P0155 may reveal or resolve other related faults. Look for systemic wiring or ECU issues if multiple O2-related codes appear.
• Safety: Work in a well-ventilated area, avoid contact with exhaust components, and disconnect the battery when performing electrical harness work as required by the vehicle's service procedure.

Summary

• P0155 indicates an O2 sensor heater circuit malfunction, with engine controller involvement and potential impact on sensor heating and emissions readiness.

• Common causes include a faulty O2 sensor heater, wiring/connector problems, and power/ground supply issues. Vehicle-specific bank/sensor mapping can vary; consult OEM wiring diagrams for exact sensor placement.

• A disciplined diagnostic approach includes visual inspection, circuit testing (voltage, ground, resistance), fuse/relay checks, possible sensor swap, and controlled replacement or repair of the faulty component.

• After repair, clear codes, drive to recheck, and verify readiness for emissions testing if applicable.

• Open Source .

• Emissions Testing and related OBD-II monitoring concepts referenced in the OBD-II literature (emissions readiness and monitor context).

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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