MPG Drop after maintenance for check engine light

Last week we took our 2001 Subaru Outback H6 in to find out why the check engine light was on.

The Independent shop I took it to claimed that the code was for a “bad battery”. New battery, terminals installed and the “check engine” light went out.

Now the fuel economy has dipped by at least 5mpg.

The repair people say they did nothing to cause this.

Anybody have any thoughts about this six cyclinder Subaru??



The Subaru H-6 engine is an extremely reliable unit. I am going to advance the theory that the removal of the battery caused the car’s onboard computer to wipe out its previous settings and to put it into a “learning mode”.

However, since you state that the battery was replaced last week, my theory would only hold water if you have not done very much driving since the battery was replaced. Is the engine idling roughly? Have you observed a drop-off in power output? If these situations have been observed, then I think that my theory is valid and that your mileage should improve within a couple of days.

Then again, this could be merely coincidental and the drop in gas mileage could correlate with lower temperatures and a change-over to ethanol “enriched” gasoline by your fuel supplier. That lovely (NOT!) ethanol-enriched gas is frequently responsible for a considerable drop-off in gas mileage.

I would sure like to know what that code number was. I can find no reference to a “bad battery” code in my Subaru code book. Did the car show signs of a bad battery? Was it turning over slowly etc.?
The check engine light went out because the battery was disconnected. I think it will come back on soon and then we can try and figure out what the problem is.

I would take a quick look around for any disconnected vacuum lines or electrical connections. It could be something simple that was disturbed when they worked on the car.

How did you measure that mileage?

I would tend to not be surprised if you did have lower mileage for a half dozen driving cycles or so. A driving cycle includes starting cold, driving until fully warm plus a few more miles. After that the computer should figure out your driving and the cars condition and have things back to normal.

Right offhand, I don’t see a battery replacement killing your fuel mileage that badly. You might swing by a local AutoZone and have them scan the car to see if there is anything in there; Check Engine Light or not. It’s free.
One would think if the mileage was really tanking like that the CEL would be on and some codes would be present.

And I’m not being snide here at all so don’t take it that way, but have you looked at the tire pressure on the chance that you may have picked up a nail. One low tire can kill fuel economy.