What is the P0055 Code?
The generic fault code P0055 indicates a resistance problem in the heating circuit of the oxygen sensor (lambda sensor) located downstream of the catalyst, on row 1 (bank 1) of the engine. This code, applicable to most vehicles since 1996, signals that the powertrain control module (PCM) has detected an abnormal resistance value in the heating circuit of the HO2S (Heated Oxygen Sensor).
Symptoms and Severity of the P0055 Fault
This code is considered serious because it directly affects the sensor’s preheating, which is crucial for the proper operation of a cold engine. Symptoms include:
- Hard starting in cold weather
- Increased fuel consumption
- Black smoke from the exhaust at startup
- Illumination of the malfunction indicator lamp (MIL)
- Risk of storing other related fault codes
Potential Causes of the P0055 Code
Several elements can be the cause of this fault:
- Damaged, cut, or corroded electrical wires or connectors
- Faulty lambda sensor (internal heating circuit fault)
- Blown fuse or cut fuse link
- Faulty engine control relay
- Problem with the PCM (less common)
Diagnosis and Solutions to Fix the P0055 Code
Preliminary advice: Check for Technical Service Bulletins (TSBs) for your vehicle model. A known fix might exist.
Recommended Diagnostic Steps:
- Visual Inspection: Carefully examine the sensor’s wiring and connectors, especially the portions exposed to heat (exhaust manifold) or friction.
- Fuse Check: Test under load all fuses and fuse links associated with the heating circuit with a digital multimeter (DVOM).
- Data Scan: Using an OBD2 scanner, read the fault codes and freeze frame data. Clear the codes and perform a road test to see if the fault returns immediately.
- Heating Circuit Test: When the engine is cold, use the scanner to observe the heating circuit voltage in the data stream. It should be close to battery voltage (approx. 12.6V). A discrepancy indicates a problem.
- Resistance Measurement: Disconnect the sensor connector and measure the resistance of the heating element with an ohmmeter. Compare the measured value to the manufacturer’s specifications.
Common Repair Solutions:
- Replace the faulty HO2S lambda sensor (most common solution)
- Repair or replace the damaged wiring harness
- Replace a blown fuse or faulty relay
- Clean oxidized or corroded electrical connectors
Important Note: After any repair, clear the codes and perform a drive cycle to allow the PCM to verify the system’s proper operation and turn off the MIL.
Conclusion
The P0055 code points to an electrical problem in your lambda sensor’s heating circuit. Although the diagnosis can be performed by a skilled DIYer with the right tools, the complexity of modern systems may require professional intervention. If you are unsure, consult a qualified mechanic for an accurate diagnosis.
Call to Action: Need help diagnosing or replacing your lambda sensor? Consult our buying guide to choose the right part for your vehicle or find a partner garage near you.