The old saying KISS - Keep it Simply Stupid

Matt Technical Support Specialist Texas Posted   Latest  

This may help some of our greener Techs on here. I put this together because I find myself talking to a lot of techs that have scatter brained diagnosis and/or missed basics. Not to say these techs are unintelligent, simply new and need direction and training.

Basic Preliminary Checks to Proper Diagnosis

As you go through the list, write down test results of every test. You may not need it now, but I promise you as your ability improves you will be pulled in multiple directions to help others. If you take good notes it will keep you from wasting time retesting what you forgot or worse missing something simple. That is the whole point of this list. I wish I could say I never put on a part only to see not a thing had changed. Every put in a PCM in a car that just needed a spark plug? I have, it SUCKS! And remember, ALWAYS DOCUMENT TEST RESULTS AND RECOMMENDATIONS! This business is all about C/Y.A.

There are many ways to skin a cat. The follow list below is what works for me. I hope it helps someone else.

FIRST - All systems DTC scan – Just that, scan every module on the vehicle and document any DTC’s, freeze frame data, monitor status, and any module(s) not communicating.

SECOND -Verify the issue – You can’t fix what is not broken. Now in some instances, like an overheat complaint, I would be doing my pressure test before verifying the issue. The reason for this is most small coolant leaks are much easier found on a cold engine and after running and engine to operating temp it will take up to or over a work day to cool to ambient (weather conditions dependent obviously). 

Intermittent issues are the most challenging issues in the industry. If the issue is not prevalent, all that can be done is form a test plan for when the issue comes back. Often and little conversation with vehicle driver(s) on information about when / how / atmospheric conditions / road conditions / etc. during the issue will reveal more info that first told that can greatly aid in diagnosis.

You will often hear that this is the first step. In my experience, I want to capture any potentially lost pending DTC or monitor info first. That is simply my personal preference.


Battery - Load test battery. Verify terminals clean, tight and have no voltage drop (see video for voltage drop test training - youtube​.​com/watch?v=YaYtCV…)

Charging System - Verify DC charging voltage is in spec with ALL loads on. verify AC voltage measured at the alternator output post is not excessive. Often manufacture’s do not publish this spec. In my professional opinion, any more than 50mV is concerning and anything over 120mV I am leaning towards alternator replacement.

Under hood -VERIFY fluids are full and is satisfactory condition. Intake piping in good condition/secure, proper FACTORY mounting, no cranks, OE air filter.

Under Driver Dash Document any potential aftermarket devices that can interfere with communication systems. I.E.; Fleet or Car lot GPS units, Breathalyzers, Aftermarket Alarm/Remote Start Systems. Often it is the installation of these system that create issues, not usually the unit itself but with installation quality. Also, we have seen case of DLC inline fleet GPS units that will have “some communication” and create unnecessary diagnostic confusion.

It is necessary in most cases to disable these systems for proper diagnosis and service. In the instance of breathalyzers, the customer should have a phone number to the service provider. Failure to proper notify the service provider and properly disarm the system could result in the customer’s arrest. I hear most people don’t pay for repair order with incarcerated, but that’s all hearsay. I personally would charge 1 hour of labor for have to do this procedure. Time is money, be efficient, charge your time honestly and accordingly.

Visual Inspection – Spending a few minutes doing thorough walk around, under hood, and under vehicle inspection can often save hours in the long run. I.E. thirty minutes into diagnostics you find a harness that has been rodent chewed.

Bulletin Search - Check for related bulletins, recall/field actions and preliminary information in repair info and in online resources, Diag​.​net, iatn​.​net, ALLDATA Community (community​.​alldata​.​com)

THEN, DIAGNOSTIC PRELIMINARY System/ Symptom Related Checks – Once the above preliminary checks have been performed and any discrepancies related to the customer concern have been addressed it then time to move on the system(s) at hand. 

I.E. an overheat issue - pressure test system, verify fan operation, thermostat operation, coolant flow, etc.

Additional Systems/Symptoms Guidance can be found here: support​.​alldata​.​com/alldata-tech-a…

Brian Diagnostician

Awesome post Matt. Good stuff 👍👍👍

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Chris Technical Support Specialist
New York

The first two steps in this post are the most important. There have been many instances where the complete vehicle scan has revealed information that was crucial to a timely diagnosis. I have received many calls with just a few codes from a single module and the missing information was found in a subsequent vehicle scan Also duplicating the concern (if possible) or at least knowing what the

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