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Diagnostic codes

So when the mechanic is talking about reading the codes, he’s talking about on board diagnostics right? So, how come my car doesn’t have a code everytime something goes wrong? Are there only certain things that have a code? Not everything has a code right?

Codes are typically recorded when you see the Check Engine Light (CEL) lit. Mostly, the term “codes” refers to emissions system errors monitored by the OBD-II computer. Brakes and transmission codes might also be stored on a different computer monitoring system. The pro mechanics can explain it better.

Ok Thanks…it’s sinking in now…

There can be lots and lots of things wrong with a car without setting any kinds of codes. Some things are just not monitored at all. As jt noted, most of them are tied to things that will produce higher emissions than the vehicle is supposed to if things are working correctly. (Of course, that does include most of the major & important engine/transmission things).

Even for things that are monitored, there is a lot of tolerance built into the monitoring. Pretty much anytime you drive your car various specs can be off here and there and from time to time. If the computer recorded a code every time something hiccuped the light would be on much of the time. There are margins of error programmed in as well as letting errors go unless they occur something like X times in X number of drive cycles/amount of time. On my own car, the last couple of times I’ve had problems that should have coded, I actually knew the problem was there, found it and fixed it before the computer ever decided to say anything about it. (On was fuel injector related and another EGR valve related).

It’s also possible for various sensors and such to be wrong - but not outside of specifications. E.g. a temperature sensor could be wrong but still sending a value to the computer that isn’t outside of what COULD be a normal reading. The computer doesn’t necessarily know any better.

So not everything has a code. For things that do have codes, not all problems will set codes - sometimes b/c it hasn’t gone outside of programmed tolerance and sometimes b/c something doesn’t “look” like a problem to the computer.

@cigroller thanks. I think as a consumer, sometimes I get frustrated when I’m left with the impression that the shop can’t help unless the car has a code…

The issue with “we can’t help if it doesn’t have a code” is frustrating and comes from a couple of things.

One is laziness and/or incompetence. A lot of people (mostly younger ones) who work in shops now actually don’t know what to do without codes. For those who do know, they might not want to be bothered. If you can spend 10 minutes with the computer diagnostics - but charge an hour’s labor which is pretty normal - and then change out a part, then you’re getting efficiencies and higher margins for yourself. If you have to spend a REAL hour doing other kinds of diagnostics then you’d actually be losing money. That’s especially the case if the problem turns out to be something cheap and simple in the end. You lost an hour and didn’t make up any money on parts.

But on the other hand, as a consumer YOU might not want to pay for an open-ended fishing expedition for problems. Having a code does greatly cut down on diagnostic time. So in the end it is often cheaper for the consumer to follow the same strategy. But it depends on the problem.

Anyway, I do recall seeing that you posted something about a car problem but I don’t recall what it was. If you are having a problem but no codes then go back to your problem thread and keep that alive as people here might be able to help.

@Jenetix…I share in your frustration as well being both a consumer and a vehicle mechanic myself. Bear in mind that not all mechanic are mechanics…some are just technicians or “wrench handlers” and those are the ones that probably need a code to help in a repair. I don’t mean to throw a wrench into things but not all codes point to the exact cause of a problem. You have to have the knowledge and experience to help find the root cause of the malfunction. A code is, in most cases, just a warning light that goes off to alert you to a problem. Repairs were made on vehicles for over 80 years before “codes” were even thought about. Mechanics in those times knew their vehicles well and repaired them in a timely manner.

The old saying says “the more you know”…so I figured it wouldn’t hurt to ask

The federal government requires all vehicles built since 1996 to have systems that monitor emissions systems and the feds have developed a standardized list of basic codes to standardize the process. The systems must store failure codes should any of the sensors detect a malfunction. Safety systems are monitored, but I’m not sure if it’s a requirement. Other systems may be monitored on many vehicles too, even though not mandated. It makes the warranty repair process much more efficient.

However, a “fault code” only tells a tech what sensor(s) detected a malfunction and what the indication was. It does not tell a tech what the exact cause is, what part needs to be replaced. It just says “I detected a faulty reading”.

But a car is a collection of complex systems, many of which work together. There are loads of problems that don’t get detected by the limited number of sensors.

One comment to my friend Missileman; where I’m from a “technician” is considered a higher skill level than a “mechanic”. A “mechanic” would be a wrench turner, a “technician” a diagnostician. But the terms are very loosely used. I have the utmost respect for a good mechanic or a good technician no matter what he/she is called. And I have the utmost distain for a dishonest or lazy one. No matter how many patches he/she has.

Yes, codes are clues that help determine what the problem is. But they do not say to change a sensor or replace another part. They are another tool a mechanic can use to determine what the problem really is.

@thesamemountainbike…I guess I was just referring to the mechanic’s world when I mentioned “technicians.” I was a missile technician in the Air Force which is a highly trained individual. A person can get hired on at a “quickie lube” joint and be a technician the next day. When I worked as a mechanic…a technician was one step above a trainee. I agree the terms are very loosely used and I meant no disrespect towards a person who is a technician. Thanks for reminding me that we don’t all live in the same neighborhood in this great country of ours.

No problem. The nomenclatures are almost irrelevant to the quality of the work. I’ve worked with engineers that were idiots and assembly line personnel that were brilliant. Unfortunately, the titles often drive the level of compensation rather than the quality of work.