Recurring Check Engine Light

I have a 1989 Toyota Celica GT convertible with 113,500 original miles. About 3 weeks ago my Check Engine light came on while I was driving. My local mechanic, who is really brilliant at diagnosing and fixing problems, and whom I trust completely, first read the computer code, # 25, which indicated that the fuel mixture was too lean. He hooked up a scope, found that the primary O2 sensor was defective, replaced it, replaced the air filter, cleared the codes, and sent me on my way. Exactly 9 days and 152 miles later the Check Engine light came on again while I was parallel parking. Again, my mechanic found that it was a Computer Code 25. He scoped the car again, tightened the large diameter hose that goes from the manifold to the air filter, extracted an aluminum plug that covers an adjustment screw (it had never been touched before), turned the screw that is near a spring gate to manually enrich the fuel mixture, cleared the codes, and sent me on my way. Yet again, for a third time, exactly one week later, the Check Engine light has come on again, this time while I was making a left turn. My mechanic pulled a # 25 computer code yet again. He is puzzled. I left the car with him overnight so he can work on it in the morning. He thinks maybe that the fuel filter needs to be changed, but wants to scope it out more fully.

Any similar experiences, and/or any ideas on what might be causing this problem?

Thank you for your help.

Cheers and Happy Holidays,


although you have implicit faith in your mechanic, does the phrase once burned twice shy come to mind?

how about once, twice, three times?

take it somewhere else. or ask your mechanic to refer you to someone else. you shouldn’t have to be paying to replace stuff on a whim, which isn’t working

A LEAN condition means that too much air, or too little fuel, is being burned in the cylinders. Too much unmeasured air can get in through a vacuum leak. A vacuum gage is used to check for partial loss of vacuum. A mechanic knows those places where vacuum can leak. Too little fuel can be the result of incorrect adjustment on the carburetor, or throttle body. A dirty air filter can lessen air flow. As can a problem with the idle air control. Low fuel pressure is another possibility. The mechanic should be refreshing his memory (for the first time?) in the particular Haynes Repair Manual for your car.

One thing that can set this code is a dirty Mass Air Flow sensor. You’re using a stock air filter in this thing right?

If the MAF sensor has enough debris coating the hot wire, it doesn’t detect the true mass amount of air entering the engine. The O2 sensor compares the amount air it detects to the amount of air the MAF sensor detects. If the O2 sensor detects a higher concentration of air compared to the amount of air the MAF sensor detects, it results in a lean condition and the Check Engine light comes on.

Have your mechanic try cleaning the MAF sensor with an aerosol MAF sensor cleaner to see if this fixes the problem.


I have an '02 Dodge Dakota (4.7 V8). Had the same problem. Took it to the Dodge dealership and diagnosed a vacuum leak. Cut the end of the hose off, plugged back in, went on my way, came on again few weeks later. Different hose, clipped and replaced. Came on again a 3rd time. Got tired of taking it in. Engine light is still on (for over a year now) and hasn’t noticeably affected anything. (Not that I’m recommending leaving it that way:) Mechanic said that because of the heat from the motor, the hoses have a tendancy to get dry and crack, producing air leaks. At some point I’ll just have them all replaced at the same time.

Toyotas of this vintage use a spring-loaded gate style MAF sensor. No hot wire. These things are a pain to diagnose, and expensive to replace. Chances are, if it is working at all, it is probably OK.

Offhand, sounds like a vacuum leak and the only additional thing I would add is that your mechanic should not be removing plugs or chipping away seals to gain access to anything.
These are sealed for a reason and if he’s dinking around with mixture? and/or idle speed screws then all he is doing is trying to override the problem, and that’s not the answer.
(And it’s not very likely the fuel filter is the problem at all.)