Error code P0138 - this is one of the most common failures in the engine management system of cars Nissan, which indicates a problem with the oxygen sensor circuit. When you see this code on the OBD-II scanner, the system reports a high signal voltage from the Bank 1, Sensor 2 oxygen sensor. In simple words, the second lambda probe located after the catalytic converter tells the control unit that the exhaust gases are too rich, although in fact the signal voltage is too high and does not correspond to reality.
Ignoring this problem can lead to serious consequences for your Nissan Qashqai, Teana or X-Trail. A faulty sensor disrupts the feedback system, which causes the ECU to incorrectly calculate the fuel mixture. As a result, you may experience increased fuel consumption, loss of power and, in the worst case, failure of an expensive catalyst due to its cells burning out with unburnt fuel.
The essence of error P0138 and the operation of the lambda probe
To understand the nature of the malfunction, it is necessary to understand the principles of operation of the emission control system. The oxygen sensor after the catalyst (Sensor 2) is not directly involved in correcting the fuel mixture, like the first sensor. Its main task is to monitor the performance of the catalytic converter. Normally, its voltage should be stable and low (usually about 0.45 Volts), since the catalyst must burn the remaining oxygen and unburned fuel.
When an error occurs P0138, the control unit records a voltage above 1.1 Volts for a certain time. This means that the signal is “stuck” at the maximum. The ECU perceives this as a signal of an extremely rich mixture, although in fact the sensor may simply fail or receive extraneous voltage. It is important to distinguish this error code from P0172 (Rich Composition), as the causes can be completely different.
In some cases, the problem is temporary and disappears after the engine warms up, but most often it is a sign of physical failure. Driving for a long time with the P0138 code can lead to overheating and complete destruction of the ceramic element of the catalytic converter., the repair of which costs several times more than replacing the lambda probe itself. Therefore, diagnosis should be carried out as soon as possible after the Check Engine appears.
The main causes of failure on Nissan
List of potential culprits for P0138 code on cars Nissan quite wide, but most often the problem lies in the sensor itself or the electrical part. The most obvious reason is the failure of the heating element or the most sensitive element of the oxygen sensor. Over time, the ceramic rod becomes contaminated with combustion products, lead or silicone, which distorts the readings.
- ⚡ Damage to the wiring: broken insulation, short to ground or short to positive in the sensor wiring harness.
- 🔋 Heater malfunction: If the lambda probe does not warm up to operating temperature, the signal may be incorrect.
- 💧 Liquid ingress: Antifreeze or oil in the exhaust system can contaminate the sensor and cause a high signal.
Problems with the catalytic converter itself cannot be ruled out. If the catalyst is destroyed or clogged, this can change the composition of the exhaust gases and affect the readings of the second sensor. It is also worth checking the tightness of the exhaust manifold: air leakage to the sensor can lead to incorrect readings, which the ECU interprets as a circuit error.
In rare cases, especially on older models, the problem may lie in the engine control unit (ECU) itself. Failures in the analog input or damage to the tracks on the board can cause P0138 to falsely trigger. However, it is worth diagnosing an ECU malfunction only after eliminating all external factors and checking the wiring.
Vehicle symptoms and behavior
The external manifestations of the P0138 error may not be as obvious as with errors of the first sensor, since this probe does not directly control the mixture. However, the driver may notice an increase in fuel consumption, especially in the urban cycle. The car may begin to operate unstably at idle, and jerks may appear during acceleration.
The main visual and audible indicator is the illuminated Check Engine light on the dashboard. In some cases, if the error persists for a long time, the "Service Engine Soon" indicator may also light up. Pay attention to the color of the exhaust gases: if black smoke comes from the pipe, this confirms the presence of a rich mixture, which could be caused by a sensor error.
Sometimes the car may not change its driving style at all, and the problem is revealed only by computer diagnostics. This is dangerous, since the driver continues to operate the car in a mode that gradually destroys the catalyst. Regularly checking for errors, even if the car is driving normally, helps avoid costly exhaust system repairs.
- Up to 100,000 km
- 100,000 - 200,000 km
- More than 200,000 km
- I don't know for sure
Methods for diagnosing and checking the sensor
To accurately diagnose the P0138 error, you will need a multimeter and an OBD-II scanner. The first step is to check the data in real time. Connect the scanner, start the engine and wait until it warms up. Look at the voltage of the second lambda probe (Bank 1 Sensor 2). It should be stable, in the range of 0.45–0.6 Volts, and should not jump, like with the first sensor.
If the voltage is constantly above 0.9–1.0 Volts, proceed to checking the electrical circuit. Disconnect the sensor connector and measure the voltage at the wiring harness (vehicle side). When the ignition is on, there should be no voltage higher than 0.5 Volts on the signal wire. If the multimeter shows 12 volts or a value close to it, it means that there is a short to positive somewhere in the wiring.
It is also necessary to check the resistance of the sensor heater. Disconnect the connector and measure the resistance between the heater contacts on the sensor itself. The normal value is usually between 2 and 10 ohms (depending on the model). If the resistance is infinite (open) or zero (short circuit), the sensor must be replaced.
- 🛠️ Check the integrity of the wires: look for abrasions in the insulation, especially in the places where the harness bends near the exhaust manifold.
- 🌡️ Inspect the connector for oxides, oil or signs of overheating.
- 💨 Make sure there is no air leakage into the intake manifold or exhaust pipe up to the sensor.
⚠️ Attention! When working on the exhaust system, make sure the engine is completely cool. Oxygen sensors are installed directly into hot metal, and the risk of getting burned during removal or installation is very high.
☑️ Checking the sensor circuit
Step-by-step instructions for replacing the lambda probe
If the diagnostics confirm that the sensor itself is faulty, it will need to be replaced. To work, you will need a special wrench for spark plugs or lambda probes (often with a slot for the wire), as well as a penetrating lubricant such as WD-40 or Liquid Wrench. By car Nissan Access to the second sensor may be limited, especially on models with two banks of cylinders or a complex subframe design.
Before starting work, generously treat the threaded connection of the sensor with penetrating lubricant. Let it sit for 10-15 minutes to allow it to penetrate the threads and break down the carbon deposits. This is critically important, since old sensors often become stuck tightly, and when trying to unscrew them, the threads in the manifold or catalyst can be damaged.
Disconnect the electrical connector and carefully unscrew the old sensor counterclockwise. If the sensor does not budge, do not use excessive force, otherwise you may break it or damage the catalytic converter. In this case, it is better to use a bolt puller or heat the joint with a blowtorch (being careful not to damage the wiring).
Before installation, be sure to lubricate the new sensor with non-stick paste, which is usually included in the kit. Never use conventional lubricants as they may damage the sensor element. Carefully screw in the new sensor, tighten to the torque specified in the instructions (usually 40–50 Nm), and connect the connector.
What is the difference between the original sensor and the analogue one?
Genuine Nissan sensors are precisely calibrated to match ECU algorithms and have a long-lasting heater. Cheap analogues may have errors in readings, which is why error P0138 may return after a short time, even with a working sensor.
Checking after repair and resetting errors
After installing a new sensor, you must reset the error. This can be done through the diagnostic scanner by selecting the "Clear Codes" or "Error Reset" function. If you don't have a scanner at hand, you can disconnect the negative terminal of the battery for 10-15 minutes, but this method is less reliable, as it may reset other settings (radio, throttle adaptation).
Start the engine and let it idle for 10-15 minutes. Make sure the Check Engine light does not come on. To be sure, you can drive about 50 km in different modes (city, highway) so that the ECU can conduct a full cycle of tests and check the efficiency of the catalyst.
If P0138 returns immediately after reset, then the problem is not in the sensor itself. There may be a problem in the wiring, the control unit, or there is a real problem with the composition of the exhaust gases (for example, a leak in the exhaust manifold). In this case, in-depth diagnostics of electrical circuits will be required.
Successful replacement of the lambda probe must be accompanied by a reset of the fuel mixture adaptations so that the ECU begins to relearn with new sensor readings.
Cost of repairs and selection of spare parts
The cost of troubleshooting P0138 depends on the method chosen and the vehicle model. Original oxygen sensor for Nissan is not cheap, the price can vary from 4,000 to 8,000 rubles and above, depending on the model (for example, for Nissan Patrol or Murano prices will be higher). High-quality analogues from the brands Bosch, Denso or NGK cost about 30–40% less, but still provide reliable operation.
If you decide to do the replacement yourself, your cost will be limited to the cost of the part. At the service center, the cost of removal and installation work will be added to the price of the spare part, which usually amounts to 1,500–3,000 rubles. Also, do not forget to include diagnostic costs if the cause was not clear at first glance.
When choosing a spare part, pay attention to the markings. Sensors for Bank 1 and Bank 2 may differ in wire length and connector type. Error P0138 refers to Bank 1, Sensor 2, so when purchasing, make sure that you need the sensor after the catalyst for the first bank of cylinders. An incorrectly selected sensor will not work correctly, even if it is working properly.
| Part type | Average price (RUB) | Service life (km) | Comment |
|---|---|---|---|
| Original (Nissan) | 5000 – 9000 | 100 000+ | Perfect compatibility, high reliability |
| High-quality analogue (Bosch, Denso) | 2500 – 4500 | 80 000 – 100 000 | Optimal price-quality ratio |
| Budget analogue (Chinese brands) | 1000 – 2000 | 30 000 – 50 000 | Risk of false alarms and rapid breakdown |
| Repair kit (tip) | 1500 – 3000 | 50 000 – 70 000 | Temporary solution, requires soldering |
⚠️ Attention! Do not try to “trick” the ECU by installing a universal lambda probe without proper calibration or firmware flashing. This can cause the Check Engine Light to stay on and cause a violation of environmental regulations, which will prevent you from passing inspection.
Prevention and operating tips
To avoid the recurrence of the P0138 code, monitor the quality of the fuel and the condition of the engine. Using low-quality gasoline mixed with lead or other additives quickly kills the lambda probe. It is also important to change spark plugs and coils on time, since misfires lead to unburned fuel entering the catalyst, which overloads the sensors.
Check the crankcase ventilation system regularly. A clogged PCV valve can cause crankcase pressure to rise and allow oil to enter the exhaust system, contaminating the oxygen sensor. Oil on the sensing element is one of the common causes of sensor failure on high-mileage vehicles.
If you plan not to use the car for a long time, it is recommended to periodically start it and let it run for 15–20 minutes. This will help keep the sensor heating element in working condition and prevent condensation and corrosion of the contacts in the connectors.
When purchasing a used car, be sure to check for P0138 with a scan tool, even if the Check Engine light is not on. The error may be “frozen” in the ECU memory, which will indicate hidden problems with the exhaust system.
FAQ: Frequently asked questions from owners
Is it possible to drive with P0138?
Technically possible, but not recommended. Driving with this error leads to improper operation of the engine control system, increased fuel consumption and the risk of catalyst destruction. It's better to fix the problem as soon as possible.
Is replacing the sensor different on different Nissan models?
Yes, sensor locations and access vary greatly. On Nissan X-Trail second generation access may be easier than on Nissan Teana or Pathfinder, where the sensors may be hidden under the subframe or heat shields.
Will cleaning the sensor remove carbon deposits?
Cleaning may help if the problem is caused by light contamination, but often sensors fail due to degradation of the internal element. Cleaning will not restore the functionality of the heater or sensing element, so replacement remains the most reliable solution.
How to check a sensor without a scanner?
You can use a multimeter to check the heater voltage and resistance as described in the diagnostic section. However, without a scanner it is difficult to track the dynamics of changes in readings in real time, which makes accurate diagnosis difficult.
⚠️ Attention! If you are not confident in your skills in working with car electrical equipment, entrust diagnostics and replacement to professionals. A connection error can lead to a short circuit and failure of the engine control unit.