O2 sensor



I’ve got a 2001 Civic with 203k miles on it. Over the last 6 months, I’ve noticed a 15% decrease in my gas mileage. The vehicle is well maintained. I keep the tires at the proper pressure, spark plugs were recently changed and 2nd timing belt service recently performed. I’ve also had a new exhaust manifold/cat converter installed due to a crack in the manifold. I’m wondering if I have an O2 sensor that is fading away but has not yet turned on a check engine light? If I were to preemptively change one of the sensors, which would have a greater impact on gas mileage the one before the cat or the one after the cat?


I wouldn’t assume that the decrease in mileage is coming from the O2 sensor. If one were off enough to cause a 15% decrease in mileage it’d be lighting the CEL.

Is this car new to you?
Has the weather been getting colder where you are?

If it were the O2 sensor, and I doubt that it is, it would be the one before the converter. That one sends a signal to the ECU as one variable in the equation that regulates you fuel metering. The one after the converter simply monitors the performance of the converter. The ECU compares its signal to the other O2 sensor and if there’s not sufficient difference in the signals the ECU lights the CEL.


I’ve had the car for several years and put most of the miles on it. In the summer, even with the a/c on 39 mpg was readily achievable on the highway. In the winter with the colder weather I could still get 36. I’m now pretty lucky to get 32 on the highway in the summer. Still good mileage by most standards but I think it should be better. Any other ideas besides an O2 sensor?


If the tires are kept in good order, and this is a new thing as you say then then I’d be likely to check the operation of the thermostat and the coolant temp sensor. If the thermostat is sticking open, even only at times, and/or the temp sensor is off you may be running too cool (or the computer may think so). This keeps the fuel mixture rich.

Presumably the air and fuel filters are also up to date?


Okay. I may look into that as well. Air filter is clean. Not sure about fuel filter.


There’s more to poor fuel efficiency issues than just the O2 sensors.
There is a whole system of parts that rely on each other to deliver the best fuel economy possible out of a motor.

First, the air has to get into the engine.
Things start at the air filter, and as you described, it appears to be clean.

Next, it passes the Mass Air Flow Sensor.
This sensor tells the engine management computer how much air is being sucked into the engine, and how fast it is traveling. If this sensor is out of spec, the engine might think that there is more air being sucked in than there actually is, and the computer will richen the mixture to compensate. It could also think the exact opposite, depending on how it is failing.

Next up are the fuel injectors. A worn fuel injector can either spray in too much or too little fuel, leak fuel, or not deliver any fuel at all.

After that it goes past the intake valves which might be covered in carbon deposits. Once in the combustion chamber, where there is most likely more carbon deposits, it has to be ignited by the spark plugs. If the spark plugs are worn out, then they won’t properly ignite the fuel mixture.

Once ignited, the fuel mixture is forced out the exhaust valves, and past the first O2 sensor, which reads how much or how little Oxygen is in the exhaust stream. If it detects too much Oxygen, the computer adds more fuel to the mixture. If it detects too little Oxygen, then the computer decreases the amount of fuel in the mixture.

The exhaust goes through the first cat convertor, and breaks up the unburned hydrocarbons and carbon monoxide, and turns them into water vapor and carbon dioxide. This converted exhaust goes past the second O2 sensor, which then confirms that the first cat is working well, and the exhaust goes past a second convertor, and then out through the tail pipe.

As you can tell, there are a lot of parts that need to work together, and it might take a lot more work to figure out which one is truly causing the poor fuel economy. My bet would be on a combination of items. MAF Sensor, O2 sensor, and spark plugs are the most likely, and with fuel injectors being very unlikely, but potential.

Good luck tracking it down.



Good post.
And you beautifully illustrated the point that there’s much more to mileage than just the O2 sensor. It’s a balance of a number of different things.

You missed the manifold absolute pressure sensor and the temp sensor as variable inputs to the ECU fuel management protocol.

This being a mileage post, I’d suggest also the ignition system. While the plugs were changed (wish they’d been read when they were pulled), any weakness in the ignition system can reduce mileage…although I’d expect a CEL with an ignition problem. It might not hurt to throw the spark pulses on a scope, though, or at least change the plug wires.

I’m going to put my money on the thermostat or temp sensor, as Cigroller suggested. My reasoning is that if the temp sensor is faulty, the ECU might bypass the O2 sensor loop longer than it should and the engine will run rich longer than it should. It might still seem to be running okay and not trip a CEL.


I agree with all the respondents. Good responses all.
The requirement of OBD 2 (On Board Diagnostics) is to turn on the check engine light when emissions would be 150% of the maximum permitted limits. The car makers aren’t telling us, in practical terms, what values from the sensors and actuators, would cause the check engine light to come on. So, one, or more, can be “off” a certain amount, and not turn on the check engine light.

You don’t have to guess if an oxygen sensor, or other sensor, is below par in performance. There are dedicated oxygen sensor testers. The better scan tools can display, on a graph, the outputs from the oxygen sensors. An oscilloscope could, also, be used to look at their waveforms.
The other sensors, like MAF, temperature, throttle, idle, have electrical values (ohms and volts) which one can measure with an electrical multimeter. Wiring diagrams help a lot, also.

Some mechanics / shops experiment with changing parts with new, customer charged parts, to see what the results in engine performance result. This, of course, is unacceptable. But, if you had some free parts to experiment with, in this manner, this could be great!


Is there a chance the transmission/clutch is slipping? 203k is a lot for a clutch or auto trans, you didn’t say which.