GAS MILEAGE: Another reason to fix the problem causing that "check engine" light!

Everyone is concerned about mileage lately. I recently talked a friend into replacing a $50 oxygen sensor that had been throwing a code for years. I told him it would pay for itself in two fill ups.

I have heard several radio ads for shops stating that if your check engine light is on, odds are your car isn’t performing as well as it should and is getting worse gas mileage than it should. They also touch on excess pollution and additional wear and tear due to the problem. Of course they mention that repairs are far cheaper than a new car right now and that keeping your car running well will extend its life. I feel this is good marketing and that one of the easiest ways to improve gas mileage is to fix known problems with your car impacting the fuel economy. There are exceptions to codes that result in lesser fuel economy but they are probably few and far between.

People would overlook the penalty in the past but gas just went north of $4 here so people are really paying attention. Spend that $50 on the sensor that has been causing a check engine light and you will save money in the long run.


I’ll just add that I replaced my cat on my riv from. Rock. It was a little over $100 if I remember right. Rusted out but still worked. But the cat for my Pontiac was $700. Take a while to make up for that but it was on Gm.

What was the outcome? How much of a fuel economy improvement?

Not everyone. Some people on this message board state that they drive as fast as they want to since their vehicles are more fuel efficient than cars 40 years ago.

I’m less sure than you are that replacing the O2 sensor will improve your friend’s mpg or even turn off the CEL, but you are right that most car owners want their car to run well, as near to like new as practical, not only b/c it makes driving more fun, but also for improved mpg and fewer emissions.

Solving the CEL problem is important b/c while it is on it may be hiding other more critical CEL messages.

1 Like

It runs smoother, with more power, and better mileage has resulted. It is too soon to know the exact amount just yet but the exhaust no longer stinks like it used to.

To add my story when I tried out my second riv with the Crt screen, I put it into diagnostics and noted that o2 sensor was lazy. The salesman didn’t know what I was doing. I bought it and had to make a 500 mile trip to sd for a funeral. I got about 12 mpg. When I got home I replaced the sensor and took it on the road. The mpg reading just kept climbing to about 27 over the 10 miles. The car was in perfect condition but I always suspected he might have traded because of the poor mileage and that may have been why the price was on the low end too.

That is funny. I know a guy who wanted some VW model and found one at the dealer. This model was known for the failure of some $1500 sensor, possibly the MAF. I don’t remember but a check engine light came on during his test drive. He took it to AutoZone and read the code and cleared it. It was the common failure that cost $1500 to fix. Anyway, he took it back after the test drive and they agreed on a deal. He wanted a 90 day warranty and explicitly requested the $1500 part be included in the wording as being covered. He played dumb and just pulled out of a bunch of stuff he printed out on the internet about the possible problem. They agreed to the terms and off he went. 2 weeks later the car was back in with the check engine light and he got that part replaced.

I once traded a chainsaw for a rifle. Then I found out after the fact that the guy who traded thought he really got one over on me. I went out and shot it the first time and couldn’t hit a sheet of plywood at 100 yards. I figured maybe the simple scope was bad so I was going to take it off and swap one from another rifle but not zero it for that gun so it could go right back on the other rifle.

I didn’t even need to do that. I started taking the “bad” scope off and one of the rings wasn’t even mounted to the gun. I fixed this on the spot and the thing shot great afterwards.

I was talking with some other guy who said he knew the guy and started telling me a story about how this guy thought he really got one over on some guy by trading his old .308 that couldn’t hit the broad side of a barn to someone for a good chainsaw. I had to share my side of the story and how it was the idiot not properly mounting the scope.

1 Like

Brought to mind an old story I told before. I wanted a small generator to run my compressor, paint sprayer, etc. off site where there is no power. I found a small one at a pawn shop for $200. It looked brand new. The exhaust was not even discolored so I bought it on condition it could run my compressor and had a day to bring it back. Got it home and it wouldn’t start. Pulled the plug and the electrode was smashed against whatever the other part is so it could not spark. It was discolored so ran at least a little at one point. I put a new plug in and ran fine. Couldn’t figure it out why and was almost like someone had tried to disable it for some reason. It’s always been a mystery to me.

Then a while back I was reading about someone that put a plug in that was too long and the piston collapsed the electrode. So my theory now is that somehow the wrong plug got put it. Started for a moment until the plug got smashed and returned it to the store. Store then dumped it on the pawn shop. I don’t remember how I determined what plug to put in and probably was the same number as what was in there unless I checked the manual. So possibly the factory got a batch of mislabeled plugs or something and put it in. So that’s my story now and I’m sticking to it. Got a brand new generator for $200.

Does that include the downstream O2 sensor?

The downstream O2 sensor is only for monitoring the catalytic converter and rarely fails.

90s Ford trucks would disable the timing advance if the check engine light came on. The fuel economy would drop to 3/4 or 1/2! It is a good way to protect the engine from possible knocking and damage. If the knock sensor is not working, and the O2 sensor is bad, there is no way to verify that the mixture is correct and a lean mixture could cause detonation and damage the valves rather quickly!

It was just the upstream sensor as that is the one that really matters. Sometimes the catalyst inefficiency code is the downstream sensor or the converter itself but that wasn’t the code he was getting. It was the upstream sensor code for his engine.

I think he isn’t regretting the $50 he spent right now.

I also replaced a battery in my car not long ago and posted about it running better. I can tell it is also using less gas. I guess that old battery was really becoming more of a liability than anything between this and the corrosion it was causing.

There’s no explanation for why that would be though. It must just be a coincidence.

Evap codes come to mind

I also suspect a oxygen sensor heater circuit code has a negligible effect on fuel economy, provided the sensor is properly switching

1 Like

I can confirm that guess. I’ve lost 3 of the 4 O2 sensor heaters on my truck at seperate times…right before a long trip each time. 2 upstream sensors, one downstream. It had no effect on the performance or mileage. Plus, 2 of the 3 times I was towing a large trailer on that trip. Just an annoying CEL glaring at me.

In my case of the bad battery, there were no check engine lights. I didn’t realize performance was suffering but could tell the car was a tad more sluggish to start. Then I noticed it spewing acid and starting to cause corrosion so I replaced it at that time. I figured being a manual I could roll start it and then replace it. My original plan was to keep the old battery as the degradation in starting was pretty gradual and change it before cold weather this fall/winter. Once I saw the leaking acid and corrosion, I decided to replace.

I immediately noticed the car running smoother. Considering this is a dinky 3 cylinder, you don’t expect a smooth idle. I figure this car doesn’t have a lot to work with so having voltage out of range was likely the issue and sensors were not properly responding.

Another possibility is something stored in the computer was wrong and replacing the battery reset to factory parameters. Either way, the battery was dying and needed to be replaced. The smoother and more powerful response which isn’t much to start with in this car plus the improved mileage is a bonus.

My 4.3L S10 once had a faulty O2 sensor and it definitely made a difference in performance and mileage. I forgot the exact code but seemed like my mileage was cut by 1/3. I know the heater circuit just gets it up to temp sooner so this won’t matter on a long trip one bit.