All modern cars come with at least two oxygen sensors. Both sensors measure the amount of oxygen present in the exhaust. These sensors provide data to the PCM for different purposes. The oxygen sensor upstream is the first sensor. This sensor is a primary input for the closed-loop fuel system. It helps the PCM regulate the air/fuel ratio of the vehicle.
Sensor 2 (also known as downstream sensor or post-catalytic convertor) monitors the oxygen capacity of the catalyst converter.
Many mechanics are unaware of a certain fact.
The PCM will set the downstream sensor to primary if it continuously receives incorrect parameters from the sensor upstream. It then adjusts fuel based on information received from the downstream sensor. P0138 is a fault code that occurs when a downstream sensor continuously sends an extremely high voltage signal (higher 1.1 volts or more) for over 10 seconds.
Row 1 is the location of the number one spark plug on the engine (applies to engines with a ‘V’ or “boxer” design). The number one spark plug can be found in row 1. Sensor 2 is located downstream of the catalytic convertor.
Table of Contents
What causes fault code P0138?
The P0138 error code can be caused by a number of different things. Most common causes include:
- Damaged connectors or cables
- Faulty oxygen sensors
- A rich engine due to problems with the fuel system
What are the symptoms of fault code P0138?
- The engine fault lamp is illuminated- Engine misfiring
- High fuel consumption
- Irregular idle
- Low power when accelerating
- Engine stalling
Your mechanic should inspect your vehicle if it exhibits any symptoms.
How to Fix Fault Code P0138?
- O2 sensor voltage values are tested. During testing, the data from both sensors must be compared.
- O2 sensor cables and connectors must be checked for corrosion and other deformations and repaired if necessary.
- Check for physical damage or contamination on the O2 sensor. The sensor must be replaced if it is defective.
- Check for engine oil and coolant leaks which could contaminate O2 sensors.
- Leaks of exhaust upstream from the sensor are examined.
- Check for any damage or obstructions in the catalytic converter.
- Replace any injectors that are faulty or leaky.
- For further diagnosis, specific tests are conducted.
How Important is Fault Code P0138?
PCM is unable to control the ratio of fuel and air in the engine. This can cause the catalytic convertor to become clogged, spark plugs to foul and an excessive buildup of carbon in the engine. This will have an impact on engine performance and comfort. The fault must be corrected immediately.
