Diagnosis 101

[pauleyman]

I wrote this years ago and thought I would share this here......(9/30/2023 thread has been updated multiple times over a decade. Please read the WHOLE thread)

These thoughts are of course, my opinion. Your results may vary.
I see a growing trend of folks who can't diagnose anything. Doesn't matter if it's a car or a toaster, they simply do not know where to start. Maybe I'm weird and took everything apart when I was growing up. I would think that to be able to diagnose what is wrong with something you should have some amount of common sense and a basic understanding of physical things, mechanical relationships and maybe some understanding of physical phenomena. In the interest of safety I'll mention a few
1. Don't put your finger close to moving parts.
2. Does the part look hot? Don't put your finger on it.
3. Did you just apply heat? It's probably still hot, don't put your finger on it.

If you are in the least puzzled or didn't know the above statements then at this point you probably have no business diagnosing mechanical things. Maybe you're more into biology and can tell me why my blood pressure is high.

Anyway, let's talk about diagnosis.

You have some mechanical/electrical item. We'll use the term broken but that can also mean it simply isn't doing what it's designed to do. Be careful here as how do you know it isn't working? Can you physically see it? Did you measure it? Or do you just know the mechanical item you are looking at isn't functioning so you ASSUME a particular item isnt' working. So now then.....

Start off with what you KNOW. Not what you think you know but what you actually know.
Your tire if flat, well how do you know? Duh you can see it.
There are tons of other things you can't see so how do you KNOW it's wrong? The best way is to measure something but if that's not possible or practical we can certainly gather some data based on other facts. There may be some assumptions along the way but no pure wild ass guesses.
Let's rule out some items
Was the item working 100% before it broke? No? 100 other things could be wrong. Good luck on your search.
Yes? Great maybe we can focus on just a few areas then.
Did you just work on a particular item/area? Yes? Start there first.
Did you see or do you see something physically wrong and you are certain of what you see? Large holes in the sides of formerly solid items are a good indication. Yes? Start there.

The point of all this talk is to gather what you KNOW so you can forms a decent diagnosis.
Try very hard to leave subjective things out of the discussion.
"It's making a funny noise" is largely a subjective statement.
"The sound the item makes has changed pitch and I have never heard it sound like that". That is much more objective and descriptive.

This analogy works fine for most diagnostic issues.

My toaster doesn't cook the toast all the way.
Did it work yesterday? Yes?
It's obviously plugged in or it wouldn't cook at all...so rule that out.
Did you check the control knob. No changes? Did you try and turn it up anyway? Still doesn't work?
Can you see the heating elements heating up?
Get the point? Look at what you KNOW.

Gather some data on what you're looking at. Is this a complicated system? Large generator perhaps? I hope you arm yourself with some technical data, specifications etc. There are usually manuals for these items, hint hint. Do you always need a manual? Of course not. That is entirely dependent upon your memory and skill level of items over time. Are you likely to remember the proper torque setting of a head bolt on a 71 nova? Not likely, that's why we look stuff up. Can you remember the spark gap on the same car? Yeah, you just might.

Even if you can't diagnose something yourself you can gather objective data and relay that to people you know or gather information form technical publications which may point you in the right direction based on what data you have gathered.

Specifically on the subject of DSMs I can tell you this. 99% of the things I have done on these cars was largely as a result of something breaking, me reading on the subject and me doing the work. When the cars were new there simply wasn't as large a pool of people to ask. I couldn't afford to pay somebody to fix it so I did it myself. I did have a good understanding of cars already so that helps. My first timing belt job, transmission rebuild, strut change etc were all done with nothing more than determination, hand tools and a shop manual. My only advantage to somebody new is I have see the same problems over and over and over so it makes it that much easier to diagnose certain things as we can make certain assumptions based on prior experiences. Again, no wild ass guesses but instead educated guesses based on past failures. Am I willing to share? Of course..... that's what we do

 

[4g63tc]

Nice write-up.

Troubleshooting is a Skill, some people have it, and some don't, Much like common sense.

The K.I.S.S. principle works for almost every situation. (Keep It Simple Stupid)

 

[pauleyman]

This deserves a bump I think.

Anytime you are trying to diagnose something start with what you KNOW. Sometimes visual inspections aren't enough. Is that lug nut tight? Well....it's seated against the rim. You know that but do you KNOW it's tight? Versus I put a torque wrench on it then YES, you KNOW objectively that it is tight.
This works for lots of stuff. Did you look at it? Or did you put a wrench on it? Did you clean it? touch it? take it apart and put it back together? scuff it? measure it? wiggle it? spray it with something? What did you DO to it to ensure you have the information you KNOW and not stuff you ASSUME?
Be objective with your diagnosis.

 

[steve]

Good stuff. When it come to diagnosing things I've learned a couple of simple rules.

Don't trust anything that happened before you got there, verify it.
Don't change more than one thing at a time or you won't know what did what.
If it didn't fix the problem put it back the way it was.
Once you do fix it, make sure by backing up a step and check it's broke again.

Most importantly, think about what you trying to prove and how what your doing will test your idea one way or another.

 

[pauleyman]

Time for another update.

For the love of GOD if you car has a check engine light start there.

Get the code, check that circuit or system. Do NOT assume a sensor is bad. As stated above look at what you KNOW. Do not assume you have a bad xyz sensor just because you have a CEL for that sensor. Check out everything. Only for the advanced should you ignore a CEL as you may know why it's there and what systems it affects.
Example...The car is breaking up at WOT only and you have an o2 sensor code. Doesn't really matter as the ECU doesn't look at o2 at WOT. Now this does NOT mean you don't have an issue transitioning from part throttle to WOT because of the o2 code. See what I mean?

For what it's worth some codes will change parameters in the ECU and some others are sort of information only. I would assume all codes cause issues. If you have a code you have a problem. Fix it.

One last thing, not all codes set a check engine light. Check codes anyway without a CEL. You could also have a burnt out bulb and unless you're paying attention when you start the car you wouldn't know.

BTW the ECU lights the CEL for 5 sec every time you start the car. That's a good indicator ECU is at least powering on. (and that the bulb works - ed)

 

[pauleyman]

Time for another update/anecdote.

My teenager has found himself with a problem on his 2011 Subaru Outback sport 5 spd. In the interest of diagnosis I shared with him the principles described above. Alas he has yet to fix it largely because he won't follow a clear path. Car has been lean and is flagging a lean code. I told him there are a few possibilities and to start with what you KNOW. Instead he keeps guessing instead of turning the wrench etc....

So, the point of this update.
GATHER INFORMATION and do NOT throw parts at the car. I hear some folks say this about their "diagnosis". I've seen it on this board recently. I replaced a,b,c,d etc and the problem persists. I'm guessing in most of those cases a,b,c,d were not bad and likely weren't checked to see if they are bad. This is a foolish and expensive way to solve a problem.

It's faster and cheaper to get the diagnostic tools needed. Almost everything I've done were with hand tools, a simple multimeter and a 25$ bluetooth ECU adapter.

In this situation he wanted to throw a new fuel assembly in (600$!) without ever diagnosing fuel pressure. I vetoed that idea but I did give him a new fuel pump to install as it was easy, free and wasn't a big deal. I do not recommend this way but it taught him something for 0$. I would've put a fuel pressure gauge on it first. It didn't fix the problem.

We are going to measure fuel pressure because the filter is in the tank and that may still be it. Next we will go after controls. He swears he cleaned grounds but did he measure the resistance? That's the important part. We will see what happens.

Coincidently, I had the exact same problem on my 2004 Outback 2 mos ago. I did check fuel pressure and no issue, so I started looking at control. What I should have done was measure resistance, but I observed some crusty connections so I went ahead and took battery out and cleaned it up and cleaned up the opposite end ground at the chassis. I then cleaned up both end of every ground wire I could find under the hood. Problem solved.

Had I measured things and been objective I am guessing I would have found poor ground connections which likely affected sensor/ECU function. FYI this did not just reset ECU which would of course reset fuel trims etc and "problem solved". It's been 2 months and there has been no change. I suspect I have other issues but when needed I will MEASURE things and base decisions on what I KNOW and OBSERVE.

Hoping this thread helps somebody.

If you've read this far you should have noticed these things.
MEASURE
OBSERVE
VERIFY
GATHER INFORMATION (Get a factory manual?)
BE OBJECTIVE NOT SUBJECTIVE

 

[CKOPOCT]

Awesome wisdom, @pauleyman! Mechanics used to be diagnosticians... but with the arrival of OBD, so many "mechanics" have allowed the computer to take the place of critical thinking.