P0138 Code: What It Means and Why It's Not Always the Oxygen Sensor

Check Engine Light Series: Article #2

Your check engine light is on and the scanner says P0138. That code means your car's computer saw a voltage signal from the rear oxygen sensor on bank 1 that stayed too high for too long. A worn sensor is a common cause, but it isn't the only one, and buying a sensor based on the code alone is one of the most common ways people waste money on this repair.

Here's what P0138 means, what you might notice, what the repair usually costs, and how we track down the real cause at our Libertyville shop.

What does P0138 mean?

P0138 stands for "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)."

• Bank 1 is the side of the engine with cylinder number 1. On a 4-cylinder engine there's only one bank.
• Sensor 2 is the downstream sensor, the one mounted after the catalytic converter.

The front (upstream) sensor helps the computer set the air and fuel mix. The rear sensor mostly watches how well the catalytic converter is doing its job. When the rear sensor's voltage reads high for a set amount of time, the computer stores P0138 and turns on the check engine light.

How oxygen sensors work and how long they last

Threaded into your vehicle's exhaust pipes or exhaust manifold is a heated oxygen sensor (HO2S), which monitors your exhaust to see if the engine is running on too much fuel and not enough air or vice versa. It continuously sends signals back to the engine control computer to adjust the amount of fuel and air going into the engine cylinders. This helps to reduce exhaust emissions.An HO2S is designed to function efficiently for 90,000 to 120,000 miles, depending upon the automaker. Once the oxygen sensor stops performing as intended, the vehicle computer detects that and turns on the Check Engine light. This will usually have a diagnostic trouble code (DTC) of P0138.That's why a lot of shops replace the sensor as soon as they see the code. Sometimes that's the right call. Often it isn't, which is what the rest of this post is about.

P0138 symptoms

Many drivers notice nothing besides the light. When there are symptoms, they can include:

• Check engine light on
• Worse gas mileage
• Rough idle
• Engine misfire
• Slow acceleration
• Stalling

If your check engine light is flashing, that's a different level of problem. A flashing light points to a severe misfire that can damage the catalytic converter quickly. Slow down and get the car looked at right away.

Can I keep driving with a P0138 code?

With a steady light and a car that drives normally, you can usually get it to the shop without a tow. The Illinois EPA's guidance is that a lit check engine light means you need service soon, not that you have to stop the car on the spot.

Don't put it off for long, though. A rich or lean condition left alone can damage other sensors, the ignition system, and the catalytic converter, which is usually the most expensive part of the emissions system to replace. And if your car is due for its Illinois emissions test, a stored emissions code with the light on is a fail. See our post on Illinois emissions testing in Lake County for how that works.

The Case for Replacing the Oxygen Sensor as Preventive Maintenance

While the vast majority of motorists and auto shops replace an HO2S only when it triggers the Service Engine Soon light, there's a case to be made for replacing it as part of a preventive maintenance routine.Why is that? An oxygen sensor with more than 100,000 miles on it doesn’t respond as quickly as it should. In the short term, that means diminished fuel efficiency, which means you're spending more at the gas pump. Sure, this won’t add up to much at first. But over the long term, a sluggish oxygen sensor has a detrimental effect on other, more expensive components.When an HO2S fails to respond efficiently, your engine will be firing with too much fuel and not enough air or vice versa. In most cases, excessive fuel that is not burned by the engine is initially deposited in the intake and cylinder heads. This fuel eventually makes its way into the crankcase, where it contaminates the oil and hinders lubrication. Lack of engine lubrication can shave years and thousands of miles from the life of an engine.Left unchecked, this rich or lean condition can also damage other emissions sensors, cause ignition system problems, ruin the catalytic converter or lead to internal engine failure.At that point, what would have been a sensor replacement has mutated into a problem that may cost thousands. Remember that your vehicle is a lot like the human body: You can’t feed it junk food, booze and cigarettes, never exercise, and expect to be healthy.

How much does it cost to fix P0138?

It depends on what's actually causing the code, which is why the diagnosis comes first.

• Diagnosis at Auto Lab: Our Level 1 check engine light diagnostic is $149. We pull the codes and then do electrical and component testing, up to an hour, to pinpoint the problem. See our pricing page.
• If it is the sensor: RepairPal's national average for an oxygen sensor replacement is $440 to $523 (about $93 to $136 labor and $347 to $387 parts), before tax and fees. Your price depends on the vehicle and which sensor it is.
• If it isn't the sensor: a wiring repair, an exhaust leak, a fuel pressure problem, or a software update all cost something different. We give you a written estimate before any work starts.

Other things that cause P0138

While it's common for the HO2S to wear out over time, there are a number of other issues that trigger a P0138 diagnostic code and have nothing to do with the oxygen sensor. These include:

• An inefficient catalytic converter.
• Engine vacuum leaks that create a lean exhaust condition.
• Mass airflow sensor failure that creates a rich exhaust condition.
• Manifold air pressure sensor failure that creates a rich exhaust condition.
• Exhaust leaks that contribute to a lean exhaust condition.
• Ignition misfires that lead to a rich condition.
• Leaky fuel injectors that create a rich condition.
• A defective fuel pressure regulator.
• A blown fuse.
• Burned or damaged wiring and/or connectors.
• Engine computer software that needs an update.

As you can see, the list goes on and on. This is why we always recommend a complete diagnosis. We hate to see situations where a vehicle owner tries to save a few bucks by having the code pulled for "free" somewhere and rushes out to buy an oxygen sensor just because it is listed in the trouble code description. The part is replaced, but the problem persists.

What To Do When Your 'Check Engine' Light Comes On

When your Service Engine Soon light turns on, you won't know whether it's because of a P0138 or a completely different issue. The first thing we advise is confirming that it is the 'Service Engine' or 'Check Engine' light. If you're not sure, consult your owner's manual.If it is the engine light, see if the gas cap is to blame. Check to make sure the gas cap is tight and that there's no rust or debris preventing it from sealing. If that all checks out and the light doesn't turn off within the next several miles, you'll want to schedule an appointment with Auto Lab for further testing. We'll be able to retrieve the code(s) and perform pinpoint testing to zero in on the actual culprit. From there, we can provide an accurate cost estimate for the repair. A parts store code read tells you the code, not the cause. Our check engine light diagnostics go further and test the circuit, the sensor, and the systems around it.See our detailed guide on how to respond when a dashboard warning light turns on.Have questions about a check engine light or need to schedule an appointment? The team at Auto Lab is always here to help.

Get your P0138 diagnosed in Libertyville

If you're in Libertyville, Mundelein, Vernon Hills, or nearby, bring it to us.

Auto Lab Libertyville
400 S. Milwaukee Ave., Libertyville, IL 60048
Monday to Friday, 7am to 6pm

• Level 1 check engine diagnostic: $149 (see pricing)
• Most diagnostics are done the same day
• Free loaner cars (first come, first served) and a free shuttle within 10 miles
• Repairs backed by our 3-year/36,000-mile warranty

Schedule an appointment or request a free estimate.

Related reading

• Check engine light codes P0300 to P0312: engine misfire
• EVAP leak behind your check engine light
• Exhaust system repair