Car computer codes can be maddening.
... they printed out about 20 things it COULD be and the total cost to check each one would add to a few hundred dollars unless they got lucky and found it quickly.
... Never had a symptom I could identify.
YMMV
How is that different from
not having the codes?
If I have a symptom, there can be many causes, sometimes hard to isolate. If you sense some misfires, could be anything in the ignition system, (plugs, wires, coil - or points, condensor, dwell, timing, or timing advance in the old days), fuel system ( fuel pump, any hose, fuel injector, air filter - or about a zillion things in the old carburators in the old days), a vacuum leak, a bad valve(s)?
The codes often are very specific. I had a code on my Volvo - engine not coming to temperature fast enough. I had a temperature gauge on the dash, and was accustomed to watching it (as a guide to turning on the heat in winter), and I could sense no difference. But the computer is sensitive enough to detect this.
The most common issues were a bad sensor and a bad thermostat. Both are easy to replace, the thermostat was cheaper, and I could get one that day at a local auto store. That fixed it.
If you are going to say
"Why fix something if you can't detect any problem with the performance of the car?", there is a simple answer - "The Environment".
All these pollution controls on cars do a fantastic job of reducing emissions, IIRC, some pollutants are reduced by
thousands of times over the cars in the 60's. For that level of operation, everything has to be "just so". Any little offset can throw the emissions way off. And as some have said, an otherwise undetected problem may ruin your Catalytic Converter, costing $$$ and creating more pollution.
I'll take the codes. They are great, but like any diagnostic tool, they aren't magic. A volt meter or an oscilloscope doesn't tell me what the problem is either. I've never had a 'scope tell me "Replace Q302 and C104". But they are valuable for finding the problems. Codes are just a tool, not magic.
-ERD50