Paying the Mechanic - for a correct diagnosis or parts and labor?

This is a recurring question that I have never resolved. It came up when I had chronic problems with diagnosing a CEL that coded as an engine misfire on my 02 Subaru OB. The dealer’s service center kept recommending, and I kept agreeing to, various different fixes that never worked (such as new cat converter, new O2 sensor, new fuel injector, new ignition coil … latest recommendation being new electrical harness). Over time, I would haggle with them and I would tend to cover one fix, while they would pick up the cost of the next. But it got me to thinking about whether it was appropriate for me to be paying for good parts and labor that, once the work was performed, turned out to be an ineffective solution. Should the mechanic pick up the tab on the next attempt to fix a problem if it turns out that what he recommended and what was done was ineffective? Like a surgeon who performs flawless surgery but for the wrong thing? Or does the owner take the risk of a mis-diagnosis, albeit one done in good faith? Any thoughts?

That’s a tough question to answer. Not all car problems are black and white issues; many of them are gray. Very gray. This is especially true of any older and/high miles car.

Without knowing the tiny details behind your problem I can only state what I often do.
When diagnosing an engine performance problem a compression test is always Step One. Always, This means even on a 1 year old car with say 20k miles on it. It should never, ever be assumed that because a car is near new that it doesn’t have a serious underlying problem related to a mechanical fault in the motor.

That being said, it could be that your Subaru may have tight valve lash on the affected cylinder. I’m assuming here that it’s a cylinder specific misfire based on the injector replacement.
Most car makers who use mechanical lifters (Subaru, Honda, etc) recommend valve lash checks at well over a 100k miles. This is a very bad recommendation. It should be done every 30k miles.

Now more bad news. If this does turn out to be a tight lash problem (on an exhaust valve especially) it could be that the car is heading for cylinder head work, as in a valve job. Tight lash on the exhaust valves will burn them quickly or damage them enough that properly adjusting the lash will only be a temporary fix.
Hope that helps and good luck.

Thanks for your response but I was offering the Subaru mishap as just one example of a general issue that I have seen. Let me give you another example to illustrate the broader issue:

I have a 1996 Jeep with 152K that stalled this past August. It had a dead battery and I was told by the mechanics that it probably had something to do with the drain associated with a loose connection on the positive terminal resulting from a cable connector that was severely corroded at the time. The mechanic dropped a new battery in and everything worked fine … for a while. Over the weekend, I encountered a similar problem – the engine turned over but stalled after several seconds and the gauges would not work – which is the subject of a post I made on the maintenance part of this forum. A reply post told me to check the positive battery cable terminal, which I did. It was the same corroded terminal connectors on a new battery. I could not get the cable off but I backed off the screw and re-tightened it and the car started and worked fine.

I drove the car back to the same mechanic shop, and they are happy to look at the problem … for a $96 diagnostic fee. If the cable terminal connector was so corroded as to risk this problem, shouldn’t the recommendation have been to repair or replace it back in August, meaning I would have avoided a second diagnostic fee in November?

That is another concrete example of what I see as a recurring problem. Any thoughts?

The mechanic should have properly cleaned the connection before reinstalling it and the entire connection coats with a protective coating after the new battery installation.

In this case, I don’t feel it was appropriate to charge you the new fee. Normally workmanship is guaranteed for one year, and this is a workmanship issue. Unless, of course, there are details not included in your post.