P0138 — O2 Sensor Circuit High Voltage #

Note: The code description may vary depending on the vehicle manufacturer.

• P0138 – Oxygen Sensor Circuit High Voltage (Bank 1, Sensor 2) — Generic OBD-II
• P0138 – Rear Heated O2 Sensor Circuit High Voltage — Honda
• P0138 – HO2S12 Circuit High Voltage — Ford
• P0138 – Lambda Sensor 2, Bank 1: Signal Too High — Volkswagen/Audi

Summary of Code P0138 #

P0138 is a diagnostic trouble code (DTC) indicating that the voltage signal from the downstream oxygen sensor (O2 sensor), located after the catalytic converter on Bank 1, is higher than the expected threshold for an extended period. In technical terms, “high voltage” typically means the sensor is reporting a rich air-fuel mixture or there is an electrical fault. This sensor, also called O2 Sensor 2 or HO2S2, monitors the efficiency of the catalytic converter, not the direct air-fuel mixture. When the Engine Control Module (ECM) or Powertrain Control Module (PCM) detects a persistent high voltage (usually above 0.9V), it sets the code and often triggers the Check Engine Light.

Common Symptoms for Code P0138 #

Drivers may notice a range of symptoms, though sometimes the vehicle may seem to run normally. Because this code involves a post-catalyst sensor, drivability can be affected but not always dramatically. Watch out for the following issues:

• Illuminated Check Engine Light (CEL)
• Poor fuel economy
• Possible rough idling or hesitation
• Failed emissions test
• Occasional sulfur or rotten-egg smell from the exhaust
• Intermittent engine stalling (rare)

Most Likely Causes for P0138 #

Several issues may trigger code P0138, but some are more common than others. The order below reflects typical frequency, from most to least likely:

1. Faulty downstream (post-catalyst) O2 sensor (worn out or contaminated sensor)
2. Shorted O2 sensor wiring or signal circuit (wiring insulation damage or corrosion)
3. Exhaust system leaks upstream of the sensor
4. Contaminated sensor due to coolant or oil burning
5. ECM/PCM malfunction (rare)
6. Faulty connector or poor terminal contact at the O2 sensor

Step-by-Step Diagnosis for P0138 #

Diagnosing P0138 requires a blend of systematic logic and hands-on checks. Here’s a proven master tech approach:

• 1. Inspect for additional codes. Always check for other DTCs, especially those related to the O2 sensors or catalyst efficiency.
• 2. Visual inspection. Examine the O2 sensor and wiring for physical damage, corrosion, or loose connectors.
• 3. Check sensor voltage with a scan tool. Monitor Bank 1 Sensor 2 readings—if consistently above 0.9V, suspect a fault.
• 4. Test the sensor circuit. With a digital multimeter, check the signal wire for shorts to power or ground.
• 5. Check for exhaust leaks. Pay close attention to connections upstream of the downstream O2 sensor.
• 6. Replace the sensor if faulty. If all wiring and circuit tests pass, the sensor itself is the likely culprit.
• 7. Clear the code and retest. After repairs, erase the code and perform a drive cycle to confirm resolution.

Possible Repairs for Code P0138 #

Repairs should always follow a confirmed diagnosis—don’t just replace parts based on the code alone. Depending on the root cause, typical repairs include:

• Replacing the downstream O2 sensor (Bank 1, Sensor 2)
• Repairing or replacing damaged O2 sensor wiring or connectors
• Fixing exhaust leaks upstream of the sensor
• Cleaning or replacing contaminated sensors if oil or coolant fouling is present
• In rare cases, replacing or reprogramming the ECM/PCM

Is It Safe to Drive with Code P0138? #

Usually, yes—for a short period. Code P0138 rarely causes immediate drivability issues because it involves the post-catalyst sensor. However, prolonged driving with this fault can mask other emissions problems and potentially cause the vehicle to fail emissions tests. If the engine runs obviously rough, or if other codes are present, limit driving and address the issue promptly. For peace of mind and to protect your catalytic converter, fix the fault as soon as you can.

Most Commonly Affected Vehicles by P0138 #

P0138 is a universal OBD-II code, so it can appear on any modern gasoline vehicle, especially those with higher mileage or in harsh climates. However, some models are more frequently reported with this issue:

• Honda Accord, Civic, CR-V
• Toyota Camry, Corolla, RAV4
• Chevrolet Silverado, Malibu
• Ford F-150, Focus
• Nissan Altima, Sentra
• Volkswagen Jetta, Passat

Frequent Mistakes & Best Practices for P0138 #

Don’t let haste or guesswork lead you astray—these are the most common pitfalls and what you should do instead:

• Replacing the O2 sensor without confirming wiring or circuit integrity
• Ignoring possible exhaust leaks or contamination sources
• Overlooking additional codes that may be related or point to a different root cause
• Not performing a thorough visual and voltage test before replacing parts
• Neglecting to clear codes and verify repair success after service

Quick FAQ for Code P0138 #

Here are fast answers to questions technicians and drivers often ask:

• Q: Will replacing the O2 sensor always fix P0138?
  A: Not always. Sometimes the issue is in the sensor’s wiring, connector, or an upstream exhaust leak. Always diagnose before replacing parts.
• Q: Can a bad catalytic converter cause P0138?
  A: Rarely. The code specifically points to the sensor or its circuit. However, a failed catalytic converter can sometimes lead to abnormal sensor readings.
• Q: Is it safe to clear the code and keep driving?
  A: Clearing the code without repairing the fault may temporarily turn off the warning light, but the problem (and potential emissions risk) remains. Fix the root cause for best results.