Can I drive with a P0138 code?

You can usually drive with P0138, but fuel economy may suffer and you will not pass emissions testing. The code indicates a sensor fault, not an immediate safety hazard, but prolonged incorrect fuel trim can damage the catalytic converter over time.

Will P0138 clear itself?

P0138 may clear itself if the fault was intermittent, but it will return if the underlying problem persists. The ECM requires multiple drive cycles without the fault detected before it clears the code and turns off the check engine light.

How much does it cost to fix P0138?

Repair cost depends on the cause. A downstream oxygen sensor typically costs between parts and labor, while wiring repair or addressing fuel contamination can be more involved. Diagnosis by a qualified technician will identify the specific fault and cost.

Is P0138 the same as P0420?

No. P0138 is a downstream oxygen sensor circuit fault, while P0420 indicates catalyst efficiency below threshold. A faulty sensor can cause P0420 to set incorrectly, so always diagnose P0138 first before replacing a catalytic converter.

What is the difference between Sensor 1 and Sensor 2?

Sensor 1 is the upstream oxygen sensor, located before the catalytic converter, and controls fuel trim. Sensor 2 is the downstream sensor, located after the catalyst, and monitors catalyst efficiency. P0138 refers to Sensor 2 on Bank 1.

P0138 Code Meaning: O2 Sensor High Voltage (Bank 1, Sensor 2)

P0138 means the powertrain control module has detected a high voltage signal from the downstream oxygen sensor on Bank 1 (Sensor 2), which monitors exhaust gases after the catalytic converter. The sensor signal is stuck above the normal operating range, typically above 0.9 volts for an extended period.

Bank 1 refers to the engine bank containing cylinder 1, and Sensor 2 is the oxygen sensor positioned after the catalytic converter. This sensor's primary job is to monitor catalyst efficiency by comparing its readings to the upstream sensor (Sensor 1). When the downstream sensor reads consistently high voltage, the ECM sets P0138 because the signal does not match expected post-catalyst exhaust chemistry.

What P0138 Means

According to SAE J2012, P0138 is defined as "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)." The code indicates the sensor's voltage output is stuck high—typically above 0.9 volts—when the ECM expects it to fluctuate between roughly 0.1 and 0.9 volts in response to changing exhaust oxygen content.

The downstream oxygen sensor operates differently from the upstream sensor. After the catalytic converter has processed the exhaust, oxygen levels stabilize, so the downstream sensor should show relatively steady voltage near 0.45 volts with minor fluctuations. A signal stuck high suggests the sensor is reading a rich exhaust condition that does not match reality, or the sensor circuit itself is faulty.

P0138 is a sensor circuit code, not a catalyst performance code. It is often confused with P0420 or P0430, which indicate catalyst efficiency problems. A faulty downstream sensor can cause the ECM to misinterpret catalyst performance, so diagnosing P0138 first is essential before condemning a catalytic converter.

Common Symptoms of P0138

Many vehicles with P0138 show no obvious drivability symptoms. The most common sign is the check engine light. Because the downstream sensor does not directly control fuel trim—the upstream sensor does that—drivers may not notice performance changes.

Symptoms you may observe include:

Check engine light illuminated
Reduced fuel economy due to incorrect fuel trim adjustments
Rough idle or hesitation if the ECM compensates for perceived rich condition
Failed emissions test
No noticeable symptoms in many cases

If you also see codes like P0172 (System Too Rich, Bank 1) or P0171 (System Too Lean, Bank 1), the ECM may be reacting to the faulty downstream sensor reading by adjusting fuel delivery incorrectly.

Common Causes of P0138 (Ranked by Likelihood)

The most common cause of P0138 is a failed downstream oxygen sensor. Sensors degrade over time due to heat, contamination, and exposure to exhaust gases. A sensor that has lost its ability to respond accurately will often read stuck high or stuck low.

Other causes, in descending order of likelihood:

Failed downstream oxygen sensor: Internal element degradation or contamination causes constant high voltage output.
Shorted signal wire: The signal wire in the O2 sensor harness may short to voltage (battery or charging system), forcing the sensor reading high.
Fuel contamination: Coolant or oil entering the combustion chamber can coat the sensor element, causing erratic or stuck-high readings.
Exhaust leak near sensor: A leak upstream of the downstream sensor can introduce fresh air, skewing the sensor reading.
ECM fault: Rare, but internal ECM issues can misinterpret sensor signals.

Diagnostic output is a decision-support estimate, not a substitute for professional inspection.

Diagnostic Path for P0138

Start by reading freeze-frame data with a scan tool. Freeze-frame captures engine conditions—RPM, load, coolant temperature, fuel trim—at the moment the code set. This context helps you understand whether the fault occurred at idle, under load, or during a specific driving condition.

Next, inspect the downstream oxygen sensor wiring and connector. Look for:

Damaged insulation or exposed wire
Corrosion or moisture in the connector
Wiring routed too close to hot exhaust components
Evidence of rodent damage

With the engine running and warmed up, use a scan tool to view live data from the downstream sensor. A healthy sensor should show voltage fluctuating gently between roughly 0.1 and 0.9 volts, settling near 0.45 volts after the catalyst. If the reading is stuck above 0.9 volts and does not respond to throttle changes, the sensor or its circuit is faulty.

Use a digital multimeter to test the signal wire for a short to voltage. Disconnect the sensor, then measure voltage on the signal wire at the harness connector with the key on, engine off. You should see close to 0 volts. If you see battery voltage or a high reading, the wire is shorted.

Inspect for exhaust leaks between the catalytic converter and the downstream sensor. A leak can introduce fresh oxygen, causing the sensor to read lean (low voltage), but in some cases, backpressure or contamination can skew readings high. Listen for hissing or ticking sounds and look for soot stains around exhaust joints.

Check for fuel contamination. If coolant is leaking into the combustion chamber (head gasket failure) or the engine is consuming oil (worn rings or valve seals), the sensor element can become coated with deposits. Inspect the oil for milky discoloration (coolant contamination) and check for excessive oil consumption or blue smoke from the tailpipe.

If wiring and contamination checks are clear, replace the downstream oxygen sensor. Use an OEM or high-quality aftermarket sensor that meets the vehicle manufacturer's specifications. After replacement, clear the code and complete a drive cycle to confirm the repair. The drive cycle should include a mix of idle, steady-speed cruising, and moderate acceleration to allow the ECM to re-evaluate sensor performance.

Tasks involving exhaust component removal, wiring diagnosis with a multimeter, and fuel contamination assessment often warrant a qualified mechanic, especially if you lack lift access or diagnostic equipment.

P0138 vs. Catalyst Efficiency Codes

P0138 is a sensor circuit fault, not a catalyst fault. The downstream sensor's job is to monitor how well the catalytic converter is cleaning the exhaust. If the sensor itself is faulty, it may report incorrect data that causes the ECM to set a catalyst efficiency code like P0420 (Bank 1) or P0430 (Bank 2).

A stuck-high downstream sensor can make the ECM think the catalyst is not reducing emissions effectively, even if the catalyst is functioning normally. For this reason, always diagnose and repair sensor circuit codes before replacing a catalytic converter. Replacing a catalyst when the real problem is a faulty sensor wastes money and does not fix the underlying issue.

If you see both P0138 and P0420 together, start with P0138. Clear the codes after repairing the sensor, then drive the vehicle through a complete drive cycle. If P0420 does not return, the catalyst is likely fine.

When to See a Mechanic

Diagnosing P0138 requires a scan tool capable of displaying live sensor data, a digital multimeter, and access to wiring diagrams for your specific vehicle. If you do not have these tools or are unfamiliar with electrical diagnostics, a qualified technician can pinpoint the fault more efficiently.

Exhaust work—removing and replacing oxygen sensors, inspecting for leaks—often requires a vehicle lift and penetrating oil or heat to free seized fasteners. Sensors exposed to years of heat and corrosion can break during removal, requiring additional repair.

If fuel contamination is suspected, further diagnosis may include compression testing, leak-down testing, or inspection of the cooling system. These tasks require specialized equipment and experience.

Professional inspection is recommended for accurate root-cause determination, especially if multiple codes are present or if initial checks do not reveal an obvious fault.

P0138 Code Meaning: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)