If you are reading this, the dreaded “check engine” light probably came on in your car. I hope that’s not your case, and you are reading this article just because you want to know about O2 sensors problems. Either way, I will try to answer two of the most asked questions about them.
These questions are:
Can you drive your car with a bad O2 sensor?
Should I replace all O2 sensors at once?
But first I would like to clarify some misconceptions about O2 sensors and their connection with the “check engine” light. The unfriendly yellow icon showing the silhouette of an engine is the standard symbol that OBD2 systems use to let the drivers know there is something wrong with their engines.
As this icon was standardized in 1996 with the implementation of the OBD2 diagnostic system as part of a series of laws issued by the U.S. government in an attempt to lower vehicle emissions, many people and most car user manuals relate this light with a bad O2 sensor, a bad catalytic converter, or as a warning informing users that their engines have some emission problem. The truth is that there are hundreds of OBD2 fault codes (DTCs) out there and not all of them are related to O2 sensors or catalytic converters, and some are not even related to air pollution!
Many people take the car to the shop in the belief that replacing the O2 sensors or the catalytic converter is going to fix the problem and, to make things worse, mechanics with poor knowledge about electronic fuel injection systems suggest replacing one or both O2 sensors without being sure that they are the reason why the “check engine” light is on.
Can a Blown Fuse Cause a Check Engine Light?
Now that you are warned, let’s talk about O2 sensors and what they do.
What O2 Sensors Do For Your Vehicle
These sensors, also known as oxygen sensors or lambda sensors, measure how much un-burned oxygen is present in the exhaust as exhaust fumes exit the engine.
Modern cars have two O2 sensors, one before the catalytic converter and another one after.
Engines with V configurations usually have 4 O2 sensors, a pair for each bank; 2 pre-cat sensors, and 2 post-cat sensors.
Front or Pre-Cat O2 sensors
These sensors are the most important ones for the proper functioning of the engine. The car’s ECU (Electronic Control Unit) uses their input to measure the fuel mixture. With this information, the ECU can determine if the fuel mixture is rich (it has an excessive proportion of fuel) or lean (there is less fuel than there should be) and can take actions such as increasing or decreasing the fuel injector’s pulse length to ensure that the engine is burning the air/fuel mixture as it should.
Rear or Post-Cat O2 sensors
These sensors measure the efficiency of the catalytic converter. The exhaust fumes should be cleaner after having being processed by the catalytic converter, meaning that if all is well, the readings of the post-cat O2 sensor should be significantly different from the pre-cat O2 sensor. If these readings don’t show any difference or show a slightly different reading than the pre-cat O2 sensor; it means that the catalytic converter is not working properly. A few models also use the post-cat O2 sensors’ information to make fine fuel trim adjustments.
Now that you have an insight into O2 sensors, it’s time to answer the questions that have been raised before.
Can You Drive Your Car With a Bad O2 Sensor?
The short answer is YES, you can, The ECU will turn the “check engine” light on your dashboard to let you know there is a problem, and it’s going to store an error code in the car’s computer memory fault. So far so good
Should I drive my car with a bad O2 sensor?
The truth is that the O2 sensor that matters for the proper running of your engine is the front one. Your car’s ECU relies on that sensor along with the MAF or/and MAP sensor to calculate the perfect air/fuel mixture that also happens to be the more efficient and less contaminating formula. Driving a car with a relatively rich air/fuel ratio won’t hurt your engine in the short run, but driving a car with a lean or poor air/fuel ratio can harm a car’s engine very badly.
The main reason for this is temperature; an engine running lean will run hotter than an engine running stoich (stoichiometric) or rich. Believe it or not, fuel helps to cool down the combustion chamber. An engine running too lean can raise so much heat inside that it can melt important components such as valves and pistons. To avoid risks, car manufacturers program the ECUs to go into “limp mode” when important sensors like this fail (car computers have several ways of determining whether their sensors are having reliable readings). “Limp mode” is like rebooting a computer in “safe mode”.
When an engine is running in this condition, the engine will ignore the readings of the problematic sensors and will use pre-programmed settings that will allow the engine to run in a relatively safe way. Under “limp mode”, the engine computer will increase the amount of fuel being delivered to the engine and getting an overly rich mixture.
The engine may run a little rough, and you are going to notice that it’s consuming more fuel than it usually does, and you will probably feel a smell of un-burnt fuel coming out of your car’s tailpipe.
One problem about using your car in “limp mode” for a long time is that the excessive rich air/fuel mixture could ruin your catalytic converter and the excess of fuel inside the combustion chamber can “fuel wash” your cylinders.
“Fuel wash” means that the oil film from the piston rings is removed by an excess of fuel. When this happens, the cylinder walls are prone to be damaged by friction and serious damage can happen, not to mention the fact that engines running in limp mode won’t pass any State’s emissions test. In some states, if your car fails this test twice you may have to pay a fine, and you won’t be able to drive your vehicle until the problem is solved.
In conclusion, driving your car with a bad pre-cat O2 sensor is possible for a short time, but in the long run, it’s cheaper to replace the bad sensor than facing the consequences of having driven your car with a bad O2 sensor for too long.
If your rear or post-cat O2 sensor is bad, your engine is going to work fine. Your “check engine” light is going to be lit on your dashboard, but your ECU won’t set the “limp mode” on.
If your front O2 sensor and your catalytic converter are working fine, having a bad post-cat or rear O2 sensor is not such an urgent matter. The problem is that if your rear O2 sensor is not working, you are never going to know for sure if your catalytic converter is working properly unless a rotten egg or sulfurous smell starts coming out of your exhaust tip.
Even worse; if you know that your rear O2 sensor is bad, you will assume that your “check engine” light is on because of that, and you are never going to know if something else is wrong in your engine except that it’s something really noticeable. You can drive your car if your post-cat or rear O2 sensor is bad, but it’s advisable for you to fix it in time to be sure that your engine is running smooth, is healthy, and, most importantly, the “check engine” light is off!
Should You Replace all O2 Sensors at Once?
This is a question that many people ask. O2 sensors are parts exposed to extreme conditions and although some manufacturers claim that they have an estimated lifespan of about 75K miles, the truth is that not every car is driven by the same people and under the same exact conditions. Being a combination of chemical and electronic parts, they work until they start to fail. There are sensors that will last a vehicle’s lifetime and others will fail before expected.
A sensor’s health is determined by the quality of its readings; once a sensor starts failing to deliver reading in the scope of its operating range, then we can say that the sensor is bad, not before. There is a saying: “if it’s not broken don’t fix it”, and it is said that engineers’ motto is “if it’s not broken, take it apart and fix it”.
Leaving the joke aside, it’s hard to know when an O2 sensor is going to fail and modern O2 sensors are expensive parts. My advice is that you replace only the O2 sensor that is failing. The labor cost of changing two sensors at once is about the same as changing just the sensor that is failing. It’s not really worth replacing all O2 sensors at once if only one is failing.