I've got a 2005 Accord sedan EX, 4 cylinder, with 121,000 miles. When keeping my highway speed at around 65 - 70 mph I have always gotten around 32 mpg. Now, that has dropped to 26 mpg over what I am guessing is about 7 to 8 months. When I realized this, I had it tuned up (new spark plugs), and had the trans fluid changed but this did not have any effect on the gas mileage. I then got two bottles of fuel system cleaner and put them in the gas and got around 31 mpg on that tank but that has now gone back down to around 26 mpg. Also, not because of the gas mileage but because they were needed, I got new tires which also did not have an effect on the gas mileage. The car's not showing any warning lights or giving any codes. My mechanic, who I trust and have used for years, thinks the mileage has gone down because the car's gotten old. Does this sound right? Should I just be satisfied with this because the car is "old"? If not, what else should I check? Thanks for any help you can provide.

 

Nope, doesn't sound right at all, in fact, some of my cars have returned their all-time best fuel economy up in the 150,000 mile range. If I were you, and in spite of the fact that I hate "throwing parts at a problem", I'd be thinking about replacing the Oxygen sensors. Why? While I've never seen the problem first hand, I've read many reports by owners of cars from all marques where they've started getting poor fuel economy but not gotten a Check Engine Lights and no OBD codes were being thrown, and O2 sensors fixed the issue.

 

shipo - Thanks for the quick response.

If I use my local (non-Honda dealer) mechanic, do you have an idea on how much it would cost to replace the sensors? Would you replace both? Would a problem with the catalytic converter cause the same symptoms?

 

your local mechanic should be able to check the health of the O2/AF sensors before you throw any parts or money at it. FWIW, my 03 4cyl gets 31 all day long on the fwy with 180k on the clock.

 

Thanks for the reply, Jeff. Just so that I can talk semi-intelligently with my mechanic, how can he check the 02 sensor? Is there a special tool that he will need? Will he be checking the temperature or the fumes coming out of the exhaust or is it some other kind of check?

Also, from what I've read on the internet, there are two O2 sensors, one in the front and one in the back. One can can have an effect on gas mileage and the other just makes sure some of the smog burns up in the exhaust. Is that correct and if so which of the sensors affects gas mileage?

 

He should have a scan tool that can check LTFT and STFT (long and short term fuel trim) Actually, the first sensor is an Air/Fuel ratio sensor or wide band O2, that's the one that would be causing your problem more than likely...the secondary HO2S (heated oxygen sensor) is used for catalyst efficiency monitoring.

Let us know what happens.

 

I seem to recall reading that RockAuto is a site sponsor:

DENSO Part # 2349040 OE Style
$81.79

 

I'm on my way out of town on a business trip and I stopped at an Advance Auto Parts store. Using their scanner we got the following readings:
ST FTRM1 (%) 5.5
LT FTRM1 (%) 6.3 (with a down arrow next to it)

Is that enough info to say whether or not the LTFT and STFT are within an acceptable range? If they are, do you have any other suggestions on what I should look for?

 

Update: I talked with the service advisor at my closest Honda dealer (50 miles away) and he could not be convinced that an o2 sensor could be bad without giving a code. He talked me into a BG fuel induction cleaning. I had this done last week ($138) and the car runs a little smoother and seems to have better acceleration but my first check of fuel economy only showed a slight improvement (27 mpg). My local mechanic said that he can replace the front o2 sensor with one made by Denso (as recommended above) for around $110. I'll probably try that next.

 

how are your brakes? dragging brakes can also cause bad fuel mileage.