Error P0138 on Nissan: why it occurs and how to fix it

Fault code P0138 on cars Nissan - one of the most common problems associated with the engine management system. This error indicates Oxygen sensor circuit high (bank 1, sensor 2), which can lead to increased fuel consumption, unstable engine operation, or even the inability to pass inspection. Unlike other errors on oxygen sensors (for example, P0130 or P0136), P0138 often signals not a breakdown of the sensor itself, but problems in its power supply circuit or the signal “hanging” at a high level.

Owners Nissan Qashqai, X-Trail, Teana and other models with gasoline engines encounter this error especially often after 100–150 thousand kilometers. Moreover, in 60% of cases the problem lies not in the lambda probe itself, but in the wiring, connectors, or even the ECU firmware. In this article we will look at all possible reasons errors P0138, diagnostic methods (including testing with a multimeter) and step-by-step repair instructions - from the simplest cleaning of contacts to replacing the sensor or reflashing the control unit.

What does P0138 mean on Nissan?

Code P0138 stands for "High Voltage in Oxygen Sensor Circuit (Bank 1, Sensor 2)" - that is, the engine ECU records constantly high signal from the second oxygen sensor (installed after the catalyst). Normally, the voltage at the output of the lambda probe should fluctuate in the range 0.1–0.9 V, but with this error it “freezes” at the level 0.8–1.0 V or higher.

It is important to understand the differences between sensors Bank 1, Sensor 1 (before the catalyst) and Bank 1, Sensor 2 (after catalyst):

• 🔹 Sensor 1 (upper sensor) monitors the composition of the fuel mixture and adjusts it in real time.
• 🔹 Sensor 2 (lower sensor) checks the efficiency of the catalyst. It is its circuit that signals the error P0138.

If the ECU sees a consistently high signal from the second sensor, it interprets this as lack of exhaust gas purification with a catalyst (although the real reason may be different). In practice this manifests itself:

• 🚨 Lights up Check Engine (sometimes flashes).
• 🛢️ Fuel consumption increases by 10–15%.
• 🔥 The engine is unstable at idle.
• 📉 Acceleration dynamics drop (especially noticeable on Nissan with HR16DE, MR20DE, QR25DE engines).

⚠️ Attention: On some models (for example, Nissan Almera N16) error P0138 may be accompanied by code P0420 (“低效率催化转换器” - low catalyst efficiency). In this case, first eliminate P0138, since it is often the root cause.

The main causes of error P0138

Unlike errors like P0130 (sensor circuit malfunction), P0138 more often associated with external factors, and not with a breakdown of the lambda probe itself. Here is a complete list of possible causes, ranked by frequency of occurrence:

Reason	Frequency of occurrence	Difficulty in eliminating
Damage to sensor wiring or connectors	45%	Low
Signal wire shorted to "+" or "ground"	30%	Average
Contamination of the sensor with oil soot or fuel additives	15%	Low
Malfunction of the oxygen sensor itself	8%	Average
ECU firmware failure or incorrect calibrations	2%	High

In first place - wiring problems. Most often, the signal wire (usually black or gray) rubs against the exhaust manifold, melts due to high temperature, or oxidizes in the connector. In second place - closures, which may occur due to incorrect installation of additional equipment (for example, an alarm) or after body repairs.

Less common error P0138 appears due to sensor contamination. This is typical for cars with mileage over 200 thousand km, where oil enters the exhaust system (through the turbine or worn valve stem seals). In this case, the sensor becomes covered with soot and its sensitivity decreases.

How to diagnose P0138 yourself

For an accurate diagnosis you will need multimeter (preferably with a diode test function) and OBD-II scanner (For example, ELM327 or Launch CReader). Follow this algorithm:

1. Treat errors as a scanner. Make sure P0138 is the only active bug. If there are related codes (for example, P0136 or P0158), eliminate them first.
2. Check the sensor connector. Disconnect the plug from the lambda probe and inspect the contacts for oxidation or burning. Often the problem can be solved by simply cleaning the contacts. WD-40 or Contact breaker.
3. Measure the resistance of the sensor heater. At the sensor connector, find the heater contacts (usually white and black wires) and measure the resistance. Norm: 2–10 ohms. If the resistance approaches infinity, the heater has burned out.
4. Check the signal wire voltage. Connect the multimeter in voltmeter mode between the signal wire (usually gray) and ground. With the engine running, the voltage should fluctuate within the range 0.1–0.9 V. If it's stable 0.8–1.0 V - problem confirmed.

If you don't have a multimeter, you can use visual method:

• 🔦 Check the integrity of the wiring from the sensor to the ECU. Pay special attention to areas near the exhaust manifold - wires often melt there.
• 🔧 Unscrew the sensor and inspect its tip. If it is covered with white or black coating, the sensor is dirty.
• 🔌 Check the sensor circuit fuse (usually EF15 or EF18 in the block under the hood).

On Nissan with V6 engines (For example, Pathfinder R51 or Murano Z50) error P0138 may appear due to incorrect marking of wires in a tourniquet. In this case, only the wiring diagram of a specific model will help.

⚠️ Attention: On vehicles with the system Eco Mode (For example, Nissan Juke) error P0138 may occur after disconnecting the battery. In this case, it is enough to reset the ECU adaptations after driving 50–100 km in normal mode.

Step-by-step instructions for resolving the error

Depending on the cause, repair methods will vary. Below - step by step actions for every occasion.

1. Cleaning contacts and checking wiring

If the problem is oxidized contacts or damaged wiring:

1. Disconnect the sensor connector and clean the contacts WD-40 or a special cleaner (CRC Contact Cleaner).
2. Check the integrity of the wires from the sensor to the ECU. Pay special attention to areas near the exhaust manifold.
3. If the wires are melted, replace the harness or insulate the damaged areas with heat shrink tubing.

2. Replacing the oxygen sensor

If the sensor is faulty (checked with a multimeter):

1. Buy an original sensor or an analogue with the same article number. For Nissan Qashqai J10 will do 22690-4M015 (Denso), for X-Trail T31 — 22690-AL60A.
2. Unscrew the old sensor using the key 22 mm (pre-heat the collector if the sensor is “stuck”).
3. Install the new sensor, first applying graphite lubricant to the threads.
4. Clear the errors with a scanner and drive 30–50 km to adapt the ECU.

3. Checking the circuit for short circuit

If the signal wire is shorted to "+" or "ground":

1. Disconnect the sensor and ECU connector.
2. Use a multimeter to check the signal wire (gray) for a short to +12 V or mass.
3. If there is a short circuit, find the damage to the harness and repair it.

4. Flashing the ECU

If the error appears after chip tuning or resetting:

• 🔧 Return the stock firmware or update it to the latest version.
• 🔄 Reset adaptations via diagnostic scanner (for example, Launch X431).

What should I do if the error returns after replacing the sensor?

If P0138 appears again, check:

1. Quality of the ECU ground (clean the contact on the body under the block).

2. Presence of water in the sensor connector (a common problem after washing the engine).

3. Condition of the catalyst - if it is clogged, the new sensor will quickly fail.

Nissan Models Most Affected by P0138

Some models Nissan suffer from P0138 more often than others due to design features. In the table below - TOP 5 "problem" cars and their typical reasons:

Model	Engine	Common cause P0138	Recommendations
Nissan Qashqai J10/J11	MR20DE, HR16DE	Melting of wiring near the collector	Install the thermal shield on the harness
Nissan X-Trail T31/T32	QR25DE, MR20DD	Oxidation of the sensor connector	Clean contacts every 30 thousand km
Nissan Teana J32	VQ35DE	Signal wire shorted to "+"	Check the harness after repair
Nissan Almera N16	QG16DE	Sensor contamination with oil soot	Check valve stem seals
Nissan Juke F15	HR16DE, MR16DDT	Firmware failure after chip tuning	Return stock firmware

On Nissan with turbo engines (For example, Juke 1.6 DIG-T) error P0138 often accompanied increased oil consumption. In this case, first check the turbine for oil leaks in the intercooler or exhaust system.

Common repair mistakes and how to avoid them

Many owners Nissan make typical mistakes when troubleshooting P0138, which causes the code to reappear. That's what can't do:

• 🚫 Ignore wiring check. In 70% of cases, the problem lies not in the sensor, but in the harness. Always test the circuit before replacing the lambda probe.
• 🚫 Install non-original sensors. Cheap analogues (for example, Febi or Meyle) often give an incorrect signal. It's better to take Denso or Bosch.
• 🚫 Reset the error without eliminating the cause. If you just erase P0138 scanner, it will return in 10–50 km.
• 🚫 Use sealant on the sensor threads. This may damage the tip the next time you replace it.

Another common mistake is incorrect sequence of actions when replacing the sensor. For example, if you do not reset the ECU adaptations after installing a new lambda probe, the engine will run unstable. To avoid this, follow these steps:

1. Install a new sensor.
2. Erase errors with a scanner.
3. Drive 30–50 km to normal mode (no sudden acceleration).
4. Reread the errors. If P0138 did not return - the problem is solved.

Preventing error P0138 on Nissan

To avoid reappearance P0138, follow these guidelines:

• 🔧 Every 30 thousand km clean the contacts of the sensor connector with a special spray (CRC Contact Cleaner).
• 🛢️ Use quality fuel (not lower AI-95). Additives in cheap gasoline contaminate the sensors.
• 🔥 Follow catalyst condition. If it is clogged, the pressure in the exhaust system increases and the sensor does not work correctly.
• 📊 Reset regularly (every 50 thousand km) fuel system adaptation through a diagnostic scanner.

On vehicles with gas equipment (GBO) error P0138 may appear due to incorrect calibration of the gas system. In this case it is necessary:

1. Adjust the gas supply so that the mixture is close to stoichiometric (λ=1).
2. Install lambda probe emulator (if the ECU does not support working with HBO).
3. Check the condition of the spark plugs - when running on gas, they wear out faster.

For Nissan with V6 engines (For example, Pathfinder R51) it is useful to carry out once every 100 thousand km preventative cleaning of the exhaust system special compounds (for example, Liqui Moly Pro-Line Drosselklappen-Reiniger). This removes carbon deposits from the sensors and extends their service life.

FAQ: Frequently asked questions about error P0138

Is it possible to drive with P0138?

Short term - yes, but not recommended. Driving for a long time with this error leads to:

• 🛢️ Increased fuel consumption (up to +20%).
• 🔥 Overheating of the catalyst (risk of melting).
• 📉 Loss of engine power.

If the error appears for the first time, you can go to a service station, but it is better to eliminate it within 1–2 days.

How much does it cost to replace an oxygen sensor on a Nissan?

The cost depends on the model and type of sensor:

• 🔧 Original sensor (for example, Denso 234-4207) — 4 000–8 000 ₽.
• 🔧 Analogue (Bosch 0 258 006 537) — 2 500–4 500 ₽.
• 🔧 Replacement work - 1 000–2 500 ₽ (depending on the complexity of access).

On Nissan with V6 engines (For example, Murano) replacement may cost more due to the inconvenient location of the sensor.

Can P0138 appear after washing the engine?

Yes, this is one of the common reasons. Water gets into the sensor connector, which leads to:

• 💧 Short circuit of the signal wire.
• 🔌 Oxidation of contacts.

To avoid the problem:

1. Before washing, disconnect the sensor connector and wrap it with polyethylene.
2. After washing, blow out the connector with compressed air.

How can you tell the difference between a faulty sensor and a wiring problem?

Run two tests:

1. Sensor check: Disconnect the connector and measure the heater resistance. If it is out of range 2–10 ohms - the sensor is faulty.
2. Wiring check: Disconnect the ECU connector and test the signal wire (gray) for a short to +12 V or mass. If there is a short circuit, the problem is in the harness.

If both tests are normal, but the error remains, check the ECU firmware.

What should I do if P0138 appears after replacing the sensor?

Probable reasons:

• 🔌 Poor quality new sensor (especially if it is a cheap analogue).
• 🔥 Damage to the wiring during installation (for example, a frayed harness).
• 📊 ECU adaptations have not been reset.

Solution:

1. Install a known-good sensor (for example, from another vehicle).
2. Test the circuit for a short.
3. Reset adaptations via diagnostic scanner.