P0151 Code (Symptoms, Causes, and How to Fix)

When you start up your vehicle, the last thing you want to see is a check engine light. But for millions of drivers each year, that’s exactly what happens. It’s a nerve-wracking experience as you try to figure out what’s going on, but with a code P0151 you likely don’t have a major issue on your hands.

But what exactly is a code P0151, and more importantly, how do you fix it? We’ll break down everything you need to know for you here.

What Does Code P0151 Mean?

OBD-II Trouble Code P0151 Description
O2 Sensor Circuit Low Voltage (Bank 2, Sensor 1)

Just because you found out the code description doesn’t mean you know exactly what the code means. A code P0151 tells you that the voltage reading coming from one of the oxygen sensors is outside the expected range.

More specifically, it’s telling you that the voltage is too low. The O2 sensor system works with at least two different oxygen sensors, one before the catalytic converter and one after it. The oxygen sensors take a reading of the emissions coming out of the exhaust, and they convert that reading into an electrical voltage.

See Also: Bank 1 vs Bank 2 Oxygen Sensor Location

The oxygen sensor before the converter tells the computer the number of emissions entering the converter, and the sensor after the converter tells the computer how many emissions are leaving the tailpipe. These two readings combined allow the computer to measure the converter’s efficiency.

But if one of the oxygen sensors is giving off a reading that doesn’t make sense it can throw this code. This can happen if the sensors say there are more emissions before the converter or if the sensors say the converter is eliminating almost all the emissions.

Related: P0140, P0141, P0153, P0154, P0155, P0157, P0158, P0160, P0161

Symptoms of Code P0151

emissions test failure

If your vehicle has a code P0151 you’re going to have a check engine light, but there’s a pretty good chance that’s the only noticeable symptom you’re going to have. Your vehicle will fail an emissions test with this code, but there’s a good chance the emissions aren’t higher than they should be.

But if the emissions are a bit higher than they should be, it’s also possible that your vehicle isn’t getting the same fuel efficiency that it should. Unless you’re closely monitoring how often you’re filling up your vehicle and the average fuel economy you’re getting, there’s a good chance you won’t even notice the slight dip.

Causes of Code P0151

oxygen sensor

The good news is if your vehicle has a code P0151, it’s almost always a faulty oxygen sensor. If an oxygen sensor fails, it can send the wrong reading to the computer, and often it won’t be able to complete the circuit.

When that happens the system will read “low voltage,” and the result is a code P0151. But while that’s the most common reason for this check engine light, it’s not the only possible reason.

There could be a problem with the wiring for the oxygen sensors too. Any break in the wires or excessive corrosion can also lead to a low voltage reading.

Finally, while it’s unlikely, it’s also possible that a faulty powertrain component is leading to the code. This can happen if the faulty powertrain component throws off another sensor, which leads to the computer expecting a different reading from the oxygen sensor.

But while this is technically possible, it’s far less likely than a faulty oxygen sensor or a wiring issue.

  • Faulty oxygen sensor
  • Faulty oxygen sensor wiring
  • Faulty powertrain components

Is Code P0151 Serious?

While you shouldn’t ignore a code P0151, it’s typically not as serious as other engine codes. If your vehicle has a bad oxygen sensor, you should be able to continue driving your vehicle without any additional problems.

However, until you completely diagnose the problem you shouldn’t just assume the issue is a faulty oxygen sensor, even if that is the most likely culprit.

Additionally, if your vehicle has the check engine light on for a code P0151, it doesn’t have any other way to tell you if another problem comes up. You’ll think the light is still on for a code P0151, but another issue might’ve come up and you might not know about it until it’s too late.

Because of this we highly recommend fixing a code P0151 as soon as possible.

How to Fix

using an OBD2 scanner

If you have an automotive scan tool, this is the perfect time to use it. You can look at the sensor data for abnormalities and diagnose and replace the faulty component from there.

However, if you don’t have an automotive scan tool you’re not completely out of luck with this engine code. Use a multimeter and probe the back of each oxygen sensor using the “1-volt range”. You’re looking for a reading between 200 mV and 800 mV under regular operation. Before testing with a multimeter give your vehicle about five minutes to warm up so you can get an accurate reading.

If you get a reading below 200 mV, then you need to replace that oxygen sensor. When replacing oxygen sensors, we recommend replacing them in pairs. Not only do they tend to fail around the same time, but since they work together having one sensor working better than another one can give errant readings.

You also should check the input voltage at all the oxygen sensors. If you’re not getting any input voltage, the problem is likely a wiring issue. If all the oxygen sensors check out, ensure the voltage from the oxygen sensors reaches the ECU, otherwise the wiring problem is on that side of the system.

Finally, if all this checks out, it’s time to use an automotive scan tool to see what’s going on with your vehicle. At this point, there are simply too many different components to run through and test to get an accurate diagnosis of the problem without a scan tool.

Adam Mann

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