5 Symptoms of a Bad Oxygen Sensor

If you have a modern fuel injected vehicle, it is almost guaranteed to have at least one or more oxygen sensors. And, at some point, that oxygen sensor will need replaced. But what are the symptoms of a bad oxygen sensor? Some are obvious while others take a little detective work.

Bad Fuel Economy

An oxygen sensor tells the engine computer how efficient the combustion process is in the engine. The engine computer balances the needs of engine load with the incoming air volume to supply the exact amount of fuel needed at that moment. But a bad oxygen sensor means the engine computer can’t get a true read on what is happening. This usually leads to a situation where too much fuel is added to the engine and ends up wasted. You may notice a rapid decrease in fuel mileage if the failure is sudden or the excessive fuel needs may seem normal if the sensor is degrading slowly. If your vehicle is getting drastically less fuel mileage than the factory EPA estimate, it is possibly a sign of bad oxygen sensor condition.

Rough Running Engine

Another one of the symptoms of a bad oxygen sensor is a rough running engine. Without the correct amount of fuel, an engine can run poorly. A rough idle, stumbling on acceleration and misfires are all signs of a potential incorrect fuel mixture. The oxygen sensor is key to keeping the air-fuel ratio in check.

Exhaust Smells Like Gas

If your exhaust smells strongly like gasoline, then your engine is running too rich. The catalytic converter will take care of some of the unburnt fuel, but it can only do so much. The feedback loop of fuel amount injected into the engine depends on the oxygen sensor to report back how complete the combustion process is in the cylinder. If there is unburnt fuel exiting the engine, the computer will pull back on the amount of fuel going in and attempt to balance it. But, if the oxygen sensor isn’t reading correctly, the engine computer will never know it is running rich (too much fuel). That fuel goes right out the tailpipe where you can smell it.

If left unchecked, a rich fuel condition can not just damage your wallet at the gas pump, but also your catalytic converters. The material inside a catalytic converter is engineered to finish the last part of the combustion process. When too much fuel escapes the combustion chamber entering the catalytic converter, it can overwhelm the process and cause it to overheat. Eventually, the catalytic converter is unable to do its job or gets clogged.

Failed Emissions

It is pretty much a given that a bad oxygen sensor leads to failing your local emissions test if you have one. Depending on your emissions testing requirements, either a reading is taken directly at the exhaust pipe or a simple scan of the onboard diagnostic system is done. 

Oxygen sensors also help track just how well the catalytic converter is doing its job. The upstream oxygen sensor takes a reading of the exhaust gasses, which is it is when compared to the reading of the downstream oxygen sensor. The onboard computer expects to see a difference in these two readings showing a reduction in harmful exhaust gasses. If there is no reduction found, then the catalytic converter is possibly bad. But you may also have an issue with the oxygen sensor. The computer expects the oxygen sensor to give a precise reading. If the readings are not within an expected range, the check engine light will illuminate. That’s one of the major symptoms of bad oxygen sensor condition.

Check Engine Light

There are good dashboard lights and there are bad dashboard lights. A warning light to alert you to an open trunk is a good thing, but a warning light to alert you that something isn’t right with the engine is bad. One of the first signs of a bad oxygen sensor that most people see is an illuminated malfunction indicator light.

Here’s a list of trouble codes that can signal it is time to replace an oxygen sensor:

• P0130 – O2 Sensor Circuit Malfunction
• P0131 – O2 Sensor Circuit Low Voltage
• P0132 – O2 Sensor Circuit High Voltage
• P0133 – O2 Sensor Circuit Slow Response
• P0134 – O2 Sensor Circuit No Activity Detected
• P0135 – O2 Sensor Heater Circuit Malfunction
• P0136 – O2 Sensor Circuit Malfunction
• P0137 – O2 Sensor Circuit Low Voltage
• P0138 – O2 Sensor Circuit High Voltage
• P0139 – O2 Sensor Circuit Slow Response
• P0140 – O2 Sensor Circuit No Activity Detected
• P0141 – O2 Sensor Heater Circuit Malfunction
• P0142 – O2 Sensor Circuit Malfunction
• P0143 – O2 Sensor Circuit Low Voltage
• P0144 – O2 Sensor Circuit High Voltage
• P0145 – O2 Sensor Circuit Slow Response
• P0146 – O2 Sensor Circuit No Activity Detected
• P0147 – O2 Sensor Heater Circuit Malfunction
• P0150 – O2 Sensor Circuit Malfunction
• P0151 – O2 Sensor Circuit Low Voltage
• P0152 – O2 Sensor Circuit High Voltage
• P0153 – O2 Sensor Circuit Slow Response
• P0154 – O2 Sensor Circuit No Activity Detected
• P0155 – O2 Sensor Heater Circuit Malfunction
• P0156 – O2 Sensor Circuit Malfunction
• P0157 – O2 Sensor Circuit Low Voltage
• P0158 – O2 Sensor Circuit High Voltage
• P0159 – O2 Sensor Circuit Slow Response
• P0160 – O2 Sensor Circuit No Activity Detected
• P0161 – O2 Sensor Heater Circuit Malfunction
• P0162 – O2 Sensor Circuit Malfunction
• P0163 – O2 Sensor Circuit Low Voltage
• P0164 – O2 Sensor Circuit High Voltage
• P0165 – O2 Sensor Circuit Slow Response
• P0166 – O2 Sensor Circuit No Activity Detected
• P0167 – O2 Sensor Heater Circuit Malfunction

You can see from this list that the onboard computer knows when it is time for oxygen sensor replacement just by checking what it is (or isn’t) doing. 

Now that you know a few common bad oxygen sensor symptoms, you are prepared for when the inevitable happens. Your local NAPA Auto Care can handle the task of replacing oxygen sensor units or you can tackle the job yourself. You can make things easier by picking up an oxygen sensor removal tool designed just for this job. Typically, an oxygen sensor costs approximately $100 with full-service oxygen sensor replacement cost in the area of a few hundred dollars.

Check out all the sensors available on NAPAonline or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. We hope that this primer is enlightening, but if you need more information on oxygen sensors, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Wikimedia Commons.