Oxygen sensors are one of the most critical supporting parts of your engine. While sitting in the exhaust, they gather information on the engine’s combustion characteristics and the fuel mixture. A damaged or ineffective sensor will affect your vehicle’s running condition for the worse, often costing you money down the road. Rather than letting the problem persist, use the information below to diagnose the issue and rid yourself of that pesky check engine light.
What’s an Oxygen Sensor and How Does It Work?
Oxygen sensors measure the amount of oxygen in the exhaust. In most modern cars, there are two sensors per cylinder bank; one is placed before the catalytic converter and the other after. The upstream sensor relays information regarding the air/fuel ratio within the engine, allowing the engine computer to adjust its fueling. It’s also commonly called a wide-band sensor. The downstream sensor relays information on the catalyst and how effectively it cleans the dirty exhaust gasses. If you’ve ever chased down a P0420 code, it’s the downstream you’re after.
As the sensors sit directly in the exhaust, the exhaust gasses pass through the sensor elements protected by the metal outer casing you’d see when looking at the tip. The sensor portion is a ceramic tip covered in Platinum Zirconia metallurgy with an electric current running through it. As the gasses pass over the sensor, the oxygen reacts with it, causing a change in the voltage running through the tip. The tip itself is what’s called a Nernst Cell. The voltage change correlates to adjustments in the fueling within the engine computer, so the engine computer will use those readings to adjust the fueling accordingly.
For the equation to work correctly, the O2 sensors must be at operating temperature, which is well above 500°. Many oxygen sensors have a heater element that uses electricity to heat the sensor within a minute, ensuring even a freshly started vehicle with a cold engine can have the best combustion possible.
Without a functioning oxygen sensor, an engine can’t accurately determine how well it’s operating. It’ll revert to a basic fueling equation based on fuel sensors and either the MAF or MAP sensor, depending on which the engine uses. However, gasoline doesn’t have a constant energy density, and neither does the air, so the engine won’t be as efficient.
Symptoms of a Failing Oxygen (O2) Sensor
The most common symptom related to a faulty oxygen sensor is a check engine light or warning on the multi-function gauge display. Oxygen sensors are vital to engine performance and economy, so many codes relate to their functionality. Here are some of the more common ones you might see:
P0131
P0137
P0150
P0155
P0156
P0158
P0171
P0172
You’ll likely see a sudden and almost immediately noticeable drop in fuel economy along with the code. That stems from the lack of accuracy a bad O2 sensor leaves the engine with. The same goes for engine performance, overall power, and response. Other symptoms that could point to a faulty sensor include:
Poor fuel economy
Poor engine performance
Rough idle
Rich fuel smell from the exhaust system
Vehicles with multiple or divided cylinder banks can have multiple pairs of oxygen sensors. If an upstream sensor on one side has tripped a malfunction indicator, then chances are the upstream sensor on the other side is close to its failure point as well. While it may be more costly, replacing them as a pair ensures more trouble-free driving later. The same goes for a downstream sensor, too.
How to Test an Oxygen Sensor
Editors Note: "KG" in the comments below added this invaluable short instruction on how to test your sensors.
"For this, you will want to use a high-impedance DC voltmeter. Start by clamping the sensor in a vice, or use pliers or Vice-Grips to hold it. Clamp your negative lead to the case and the positive to the output wire. With a propane torch set to high, use the inner blue flame tip to heat the fluted or perforated area of the sensor. It will glow red hot, and this process will burn some, if not all, of the carbon build-up off it. You should see a DC voltage of at least 0.6 within twenty seconds. If not, the most likely cause is an internal open circuit or lead fouling. If everything is OK so far, remove it from the flame. You should see a drop to under 0.1 volts within four seconds. If not, it's likely silicone fouled. If it's still OK, heat the O2 sensor for two minutes and watch for voltage drops. Sometimes, internal connections will open when heated. If the sensor is still OK and will switch from high to low quickly as you move the flame, the sensor is good. Remember that good or bad is relative, with port fuel injection needing faster information than carbureted systems.
Any O2 sensor that will generate 0.9 volts or more when heated shows 0.1 volts or less within one second of flame removal and passes the two-minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. It will calibrate your evaluation skills and save you money.”
How Oxygen Sensors are Used for Fuel Management
Generally, your engine's ECU uses input from the upstream wide-band O2 sensor for fuel management, such as readjusting the fuel mixture for optimum emissions, fuel economy, and performance. If you have below-average fuel economy, weaker than normal engine responsiveness, and overall performance, that points to a faulty front O2 sensor. The signal from the downstream oxygen sensor is used primarily to detect any problems with the vehicle's catalyst and to tune the fuel trim.
The operating range of wide-band O2 sensors allows for faster reaction time when correcting fuel trim. It keeps the engine's operating range as close to stoichiometry (14.7:1 air-fuel ratio) as possible, if not a little leaner. In doing so, emissions on a newer vehicle are reduced significantly to comply with today's stricter emissions laws. Because of this, however, these wide-band O2 sensors are much more expensive than narrow-band O2 sensors.
The OBD II system on all modern cars should detect any faults with your car's oxygen sensors, whether related to the internal heater or the circuit leading to your sensor, and give out one or more fault codes, turning on the check engine light. If the diagnosis confirms a faulty sensor (and not a wiring issue or other engine fault that could cause a similar situation), replace the sensor ASAP.
Which Oxygen Sensor Is Best For You?
Plug your vehicle information into fcpeuro.com, and you’ll be met with a few options. They’re likely to be from Denso or Bosch and will plug directly into your selected vehicle without issue. Our catalog associates have worked hard to ensure that you don’t have to think about the purchase any more than that. However, if you’re reading this and FCP Euro doesn’t cover your vehicle, then the next paragraph is for you.
You can get oxygen sensors in two varieties: exact-fit units and universal sensors. Exact-fit sensors are your best bet for any repair. They don’t require custom work; you simply plug them in, and they’re ready to go. Universal sensors are also an option and are always cheaper than the exact fit. However, instead of an electrical plug on the end, they have a cut pigtail that requires you to solder or crimp the plug from the old sensor onto the new one. It’s not a difficult job, but it will lead to more issues if not done correctly, so buy accordingly.
