You no doubt know that when your Check Engine light (CEL) illuminates and doesn't go out again, it means that a sensor for a system somewhere in the drivetrain has registered an out-of-spec reading, storing a trouble code in the powertrain control module (PCM). You probably also know that you can quickly hook up a code scanner to your car's diagnostic connector and access the codes that are stored in the PCM.
With the advent of OBD-II diagnostic protocols in the 90s, this standardized system makes it easier to troubleshoot and nail down a problem, across all makes and models. The trouble codes can point you the right direction, but there's also a certain amount of deduction and reading-between-the-lines involved, especially when a problem sets off a string of trouble codes. It's a mistake to just replace parts related to a code without doing a full analysis of the problem. In this article, we'll look at potential causes of P0051 and how to fix them.
Note: Code scanning is one of the free services Advance Auto Parts offers.
What It Means
Your car's oxygen sensor(s) is designed to monitor the content of exhaust gases as they make their way through the system. It then sends information back to the PCM on oxygen content in the exhaust, and the PCM uses this to adjust fuel metering strategy. Since the '90s or early '00s, the oxygen sensor on vehicles has used a heater to get it up to operating temperature faster than the engine can. Getting the O2 sensor up to operating temperature quickly means that it can do its job in the emissions and fuel delivery system faster. A P0051 code means that the heater in the sensor has stopped working.
Symptoms
- Poor fuel economy
- Rough idle
- Rich-running condition or black smoke
- Illuminated CEL
- Rough-running
- In some cases, there may be no symptoms other than the CEL
What Happens If I Ignore It?
You can probably leave a burned-out O2 sensor in place for a long time if you don't mind poor fuel economy and a rough-running vehicle, but of course that's not advisable. You won't be able to pass an emissions test, and you run the risk of burning out the other O2 sensors or causing other problems down the road.
Diagnosis
In some cases, a problem with the O2 sensor heater can be due to a fuse or breaker, or frayed, chafed or corroded wiring or connectors. Usually, though, it's the sensor itself, as these components generally have a finite service life and may fail after about 75k miles.
Possible Fixes
This particular code does nail down the specific sensor that's registering a problem, so that's a good start.
- Locate the sensor in question and check its wiring and connector for fraying, damage or chafing.
- Confirm a good ground by checking continuity between the sensor and its ground. If you don't have continuity, this indicates a problem in the wiring on the ground side of the circuit.
- Use a multimeter to check voltage to the sensor — with the key in the "on" position, the meter should read around 12 volts.
- If no voltage is showing up, check the fuse or wiring. Low voltage means excessive resistance in the power side of the circuit.
- If you have the full 12 volts, go ahead and change the sensor itself because you can be pretty sure it's failed.
Pro Tip: After tens of thousands of heating/cooling cycles, the O2 sensor may be pretty difficult to remove. Most techs use a generous amount of penetrant while the sensor is still warm and then remove it with a special O2 sensor socket, after giving the penetrant plenty of time to work.
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About the Author(s)
Jerry Renshaw is a veteran journalist, gearhead and guitar player, and cultivated his mechanical skills by 30 years of turning wrenches to keep one piece of junk or another on the road. He's owned everything from a Chevette to (three) minivans to a fire-breathing Dodge muscle truck, and is constantly keeping up to speed on what's going on in the automotive world.
