OBD2 Diagnostic Code
P0138
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2) — What It Means & How to Fix
P0138 is not a fuel pump code. It is not an emergency. But if you let a shop replace your catalytic converter before ruling out a an inexpensive downstream oxygen sensor, you will regret it. This guide tells you exactly what to check first — and what to say if they push back.
Severity
5 / 10
Moderate. Not an emergency, but do not ignore it — it can mask a failing catalytic converter.
Safe to Drive?
Usually Yes
No immediate drivability danger in most cases. Will cause an emissions test failure. Address within a few weeks.
Typical DIY Fix Cost
Low
A downstream O2 sensor is inexpensive. The sensor swap is a straightforward job on most vehicles.
🔍 What This Code Means
Your engine has two oxygen sensors on each bank. Sensor 1 is upstream — before the catalytic converter — and it actively controls the air-fuel mixture. Sensor 2 is downstream — after the catalytic converter — and its job is simpler: confirm that the converter is cleaning the exhaust properly.
P0138 means your downstream O2 sensor on Bank 1 is sending voltage that is too high for too long. A healthy downstream sensor produces a low, steady voltage — typically below 0.4V because the catalytic converter scrubs most of the oxygen out of the exhaust before it reaches that sensor. When the computer sees voltage staying consistently above 0.9V, it sets P0138.
💡 Bank 1 is the side of the engine that contains cylinder #1. On a 4-cylinder engine that means the only bank — the whole engine. On a V6 or V8 it is typically the front bank or the driver's side, depending on the manufacturer.
High voltage from the downstream sensor tells the computer one of two things: the exhaust still contains unburned fuel at that point (a rich condition), or the sensor itself is contaminated and stuck reading high. Most of the time it is the sensor — not the converter.
⚡ Common Symptoms
Because the downstream sensor monitors converter efficiency rather than fuel control, P0138 rarely produces strong drivability symptoms. Many drivers only discover it during a routine scan or a failed emissions test.
Check engine light only
The most common presentation — no drivability symptoms whatsoever, just a lit MIL. The computer detected the fault without you noticing anything wrong.
Failed emissions test
Any active code turns on the check engine light, and a lit check engine light is an automatic fail in most jurisdictions. P0138 alone can kill your registration renewal.
Slightly reduced fuel economy
If the underlying cause is a genuine rich condition (not just a bad sensor), the engine may be burning extra fuel. The drop is typically subtle — a few MPG at most.
Faint fuel smell from exhaust
Unburned fuel passing through to the downstream sensor can produce a noticeable rich smell at the tailpipe. More common when the real cause is a fuel delivery or injector issue.
Rough idle (rare)
Only appears when the code has a fuel system root cause — for example, leaking fuel injectors delivering excess fuel. A bad sensor alone will not cause rough idle.
No symptoms at all
For most drivers, P0138 is completely silent. The car feels and drives normally. That is exactly why the misdiagnosis rate is so high — there is nothing obvious to investigate.
🎯 Most Likely Causes
Start at cause #1 every time. The majority of P0138 diagnoses are resolved before reaching cause #3 — and cause #5 is rare enough that it should never be the opening assumption.
1
Faulty or contaminated downstream O2 sensor (Bank 1, Sensor 2)
The sensing element ages and degrades over time, especially past 100,000 miles. Oil, coolant, or silicone contamination can also cause the sensor to stick at a high-voltage output. This is the most common root cause — a new sensor clears P0138 outright in about half of all cases.
~ 50% of cases
2
Rich-running engine (excess fuel reaching the downstream sensor)
If the engine burns too much fuel, unburned hydrocarbons reach the downstream sensor and produce a false-high voltage reading — even from a perfectly good sensor. Look for a companion code like P0172 (System Too Rich, Bank 1). Causes include stuck-open injectors, a faulty fuel pressure regulator, or a saturated evaporative purge valve.
~ 25% of cases
3
Wiring fault — short to voltage in the sensor signal circuit
The signal wire that runs from the O2 sensor back to the ECM can chafe against an exhaust heat shield or rub through its insulation over years of heat cycling. A short to a 12V source on the signal wire drives the sensor reading artificially high. Visually trace the harness along the underside for any rubbed, melted, or pinched sections.
~ 15% of cases
4
Degraded catalytic converter no longer reducing exhaust oxygen
A failing converter cannot process exhaust gases properly. Unburned fuel and hydrocarbons pass straight through, producing the same high-voltage signal that a contaminated sensor would. P0420 (Catalyst System Efficiency Below Threshold) usually accompanies this, but not always in early-stage converter failure.
~ 8% of cases
5
ECM fault or software issue (rare)
Very occasionally the engine control module misinterprets sensor data or develops a mapping error after a software glitch. Rule out every other cause first. If a confirmed-good sensor and verified wiring still produce P0138, an ECM software update or reflash is worth investigating with a dealer scan tool.
~ 2% of cases
🔧 What to Check First
These three steps cost nothing and require no special tools beyond an inexpensive OBD2 scanner. Do them in order before spending anything on parts.
1
Pull live sensor data and compare Sensor 1 vs Sensor 2
Connect your OBD2 scanner and watch both oxygen sensors at the same time while the engine is at normal operating temperature. Sensor 1 (upstream) should be switching rapidly — oscillating between roughly 0.1V and 0.9V several times per second. Sensor 2 (downstream) should sit low and mostly steady, below 0.4V. If Sensor 1 is switching normally and Sensor 2 reads stuck above 0.9V, the downstream sensor is the prime suspect — not the catalytic converter and not the fuel system.
2
Inspect the sensor wiring harness along the exhaust pipe
With the engine off and exhaust cool, get under the car and trace the wiring harness from the downstream O2 sensor (located after the catalytic converter) back toward the firewall. Look for chafe marks against the heat shield, cracked insulation from heat exposure, or connector pins that are corroded or pushed back in their housing. A quick wiggle of the connector while watching live data on your scanner can reveal an intermittent wiring fault instantly.
3
Check for companion codes before replacing anything
Scan for all stored and pending codes — not just P0138. If you see P0172 (System Too Rich, Bank 1) alongside P0138, the sensor is likely reading accurately and the root cause is a fuel delivery problem. If you see P0420, that points toward the catalytic converter. P0138 alone, with no companion codes and normal Sensor 1 activity, almost always means the downstream sensor itself has failed.
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⚠️ Common Misdiagnosis
P0138 has one of the highest unnecessary-repair rates of any O2 sensor code. These are the three most expensive wrong turns — and what to say to avoid them.
⚠ Misdiagnosis #1 — Catalytic converter condemned without checking the sensor first
A degraded catalytic converter can produce P0138 — but a failed downstream sensor produces exactly the same code with identical live data patterns. The correct diagnostic sequence is: verify sensor wiring → confirm sensor age and mileage → replace sensor → retest after 50 miles. If P0138 returns after a confirmed good sensor is installed, then pursue the converter. Any shop that opens with a converter quote on a P0138 with no companion P0420 is skipping the most important step in the process.
Potential savings: Sensor cost vs. full catalytic converter replacement
⚠ Misdiagnosis #2 — Fuel system repairs ordered before checking for companion codes
P0138 can be caused by a genuinely rich-running engine — but only when P0172 or similar fuel-trim codes accompany it. If P0138 appears alone with no fuel trim codes and no rich-running symptoms, replacing injectors or a fuel pressure regulator is treating the wrong problem entirely. Always pull the full code list before authorizing any fuel system work on a P0138 diagnosis.
Potential savings: Avoiding unnecessary fuel system work
⚠ Misdiagnosis #3 — Both O2 sensors replaced when only one is bad
Some technicians suggest replacing both the upstream and downstream sensors as a package when one code appears — on the logic that "if one is going, the other is close." That rationale might make sense at 180,000 miles with both sensors original. At 90,000 miles with only P0138 set and a normally-switching Sensor 1, you are paying for a part that is not causing a problem.
Potential savings: One sensor instead of two
💰 Estimated Repair Cost
DIY cost = parts only. Shop cost = parts plus labor. Ranges vary by vehicle make, region, and shop rate. These are planning estimates — not quotes.
Downstream O2 sensor replacement (Bank 1, Sensor 2)
DIYLow
ShopLow–Moderate
LevelEasy–Moderate
O2 sensor wiring harness repair
DIYMinimal
ShopLow–Moderate
LevelModerate
Fuel injector cleaning (if rich condition is confirmed)
DIYModerate
ShopModerate
LevelProfessional
Evaporative purge valve (EVAP) replacement
DIYLow
ShopModerate
LevelEasy
Catalytic converter replacement (only if confirmed bad)
DIYHigh
ShopVery High
LevelAdvanced
✅ In the majority of verified P0138 cases, the fix is a single downstream O2 sensor. Confirm it with live data before authorizing any other repair.
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🚗 Most Reported On
P0138 can appear on any gasoline vehicle. These models come up most frequently based on owner reports and confirmed repair records.
Toyota Camry (2007–2017, 2.4L / 2.5L)
Honda Accord (2008–2015, 2.4L / 3.5L)
Ford F-150 (2009–2016, 5.0L / 5.4L)
Nissan Altima (2007–2015, 2.5L)
Chevrolet Silverado (2007–2013, 5.3L)
Toyota Corolla (2009–2019, 1.8L)
Honda Civic (2006–2015, 1.8L)
Jeep Grand Cherokee (2005–2015, 3.7L / 4.7L)
❓ Frequently Asked Questions
Can I drive with P0138 active?
Yes, usually — but not indefinitely. P0138 rarely causes a drivability emergency. The real risk is that a stuck-high downstream sensor can mask a failing catalytic converter. Drive it too long and a small sensor replacement turns into a much larger repair bill.
P0138 vs P0136 — what is the difference?
Both codes point to Bank 1 Sensor 2 — the downstream O2 sensor. P0136 means the sensor circuit has a general malfunction. P0138 is more specific: it means the sensor is producing voltage that is too high for too long, typically above 1.2 volts. P0138 almost always points to a rich fuel condition or a contaminated sensor rather than a broken wire.
Will P0138 cause a failed emissions test?
Yes. A P0138 code means the check engine light is on, and in most states a lit check engine light is an automatic emissions failure regardless of what the code is. Fix the underlying cause, clear the code, and drive at least 50–100 miles of mixed city and highway to allow the readiness monitors to reset before testing.
How do I know if it is the sensor or the catalytic converter?
A bad catalytic converter usually produces P0420 or P0421 alongside P0138. If P0138 appears alone, start with the sensor. Compare live O2 data: the upstream sensor (Sensor 1) should oscillate rapidly between 0.1V and 0.9V. The downstream sensor (Sensor 2) should sit low and steady, typically below 0.4V. A downstream sensor reading consistently above 0.9V while the upstream oscillates normally points to a sensor fault — not a converter fault.
Can an oil leak cause P0138?
Yes. Oil dripping onto the downstream O2 sensor can contaminate the sensing element and cause it to read abnormally high voltage. If you see P0138 alongside an oil leak or a burning-oil smell, fix the leak first. Replacing the sensor without fixing the leak will produce the same code within weeks.
🔗 Related Codes
These codes share root causes with P0138 or commonly appear alongside it in a diagnostic scan.
This guide is for informational purposes only. Always consult a qualified mechanic before performing any vehicle repair. OBDPlatform is not responsible for actions taken based on this content.