"Check Engine" light - no reason found

The “Check engine” light comes on about every 200 miles. It goes away when I remove the battery terminals; lubricate them with conducting battery terminal grease; and re-mount them tightened to the nth degree but comes back 200 miles later.

I don’t like the design of those cables, but the problem is NOT loose cables.

I always ensure that the gas cap is tightened to the Nth degree.

Mechanic has not been able to solve.

You are erasing the trouble code(s) when you disconnect the battery. The recurrence of the CEL has nothing to do with the battery cables.

Time to change mechanics. You need to find out what code or codes are that are tripping the CEL. In most states, larger auto parts store will read the codes for free, but I have a feeling you won’t be performing the work yourself.


Find a shop that has a scanner that can retrieve history codes.

That’ll indicate why the Check Engine light came on.


Yes you are erasing the codes. You need a OBD2 scanner. It plugs in under the dash on the left side. It will give you the codes. More than likely its your oxygen sensor. You have to of them up stream and down stream. What year is your Ford Focus? I have worked on 2000-20012. all pretty much the same.

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Neither a new mechanic nor an old mechanic car read fault codes that have been erased.


If this was a human health case, the OP would be touching-up his X-rays and then wondering why his doctor couldn’t diagnose him properly.

  1. The car had been tested at least once - to no avail. Even so, if anything were really wrong, how long would the code stay erased?
  2. I found it - an oxygen sensor. You were right about the auto supply shops being willing to test.
  1. It depends on what the problem is. Some will pop up almost immediately. others will take longer, like 200 miles. as you have found out.

True, but if the OP told the mechanic about the battery cables being removed, he/she should have told the OP to stop doing that.


What are the diagnostic code(s)? ford/taurus/year/engine/miles?

Just curious, the CEL come on right after you fill the gas tank? I suspect that you would fill the tank about every 200 miles. If so, it is probably NOT the O2 sensor.

No, if that had happened, I would have made sure of the gas cap and seal.

It sounds like you’re trying to diagnose this yourself via a low-tech route, and that just doesn’t work anymore. There are different monitors within car’s computer for various systems, and they take different amounts of time to run. For instance, the monitor for your catalytic converter requires the car to be driven at a certain speed for a certain amount of time, before it will run - only after a monitor has run can the CEL for that monitor be illuminated.

I’m willing to bet this is something catalyst related. You say you got an O2 sensor code - which code is it? There are dozens of O2 sensor codes. Please also don’t assume this means the O2 sensor is bad - there are many things besides a bad sensor that could cause this to set.

Please find a competent mechanic to diagnose your issue. Look for an independent mechanic or the dealer, not the chain store down the street.

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Did your auto supply shop give the actual code number? Many will give you a printout from the cash register with the actual code at the top and suggested repairs listed under the code.

If you don’t have that code, next time you get a CEL, have you auto parts store read the code and write it down, then post the actual code here. You will get a LOT more GOOD advice if you do this. Someone here may have actual experience with that code on the same or similar vehicle and can tell you what exactly fixed it.