Strategy Based Diagnostics

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Table of Contents


This strategy-based diagnostic procedure was developed specifically for the Automotive Essentials Course and is meant to give an entry-level tech a solid diagnostic platform that can be expanded to include advanced diagnostic procedures in the future.

Also note that this concept was developed many years ago (20 plus) through the GM training center and still is in use today.

Step 1 – Verify Driver’s Concern

  • Before any diagnostic work is started, the driver’s concern must be fully understood by the tech.
    • If the service advisor can’t produce a detailed concern, then the tech should attempt to “Interview the driver of the car”
  • If the car is perfect except for an illuminated malfunction indicator light (MIL) move to Step 2
  • Test drive the
    • It may require driving the car in a specific way under specific conditions to duplicate the problem

Step 2 – Visual Inspection & Check For Codes

Visual Inspection

  • Perform a bumper to bumper visual inspection (Many problems are caused by a lack of maintenance)
    • What kind of shape are the tires?
    • What kind of shape is the oil?
    • In general, how well does the person take care of their car?
  • Are any hoses/wires unplugged?
  • Any newly installed aftermarket equipment?
  • Is there any wiring damage?
    • Also, look for signs of critter damage to wiring and hoses
  • Is there a trailer hitch on the car?
  • Is there any fresh body damage?
  • Is there any fresh damage under the car?

Check for Codes

  • Check for all systems for codes – even if the malfunction indicator light (MIL) is not illuminated
    • Most premium scan tools can scan all modules on a car to check for stored codes (this is sometimes called a “Health Check”)
  • Record all stored code info and related freeze frame data
    • Do not clear codes without recording this data
    • Many premium scan tools now can back up vehicle data to the internet automatically for later review
  • Make note of any p
    • A pending code from a system unrelated to the current problem could mature to a stored code (illuminates the MIL) after the customer takes delivery of the car and it could appear to them as the same problem.

Step 3 – Focus on Suspected Systems

Narrow Down to a Single System

  • In this step, try to narrow down (focus) on a single system.
  • See if there are any service bulletins that might shed some light on the current problem
    • You are not looking for a “Silver Bullet”, just supporting information
    • Even if there appears to be a “pattern failure”, test – don’t guess
  • Start reading up on “System Operation” on the suspected system.
    • Most manufacturers do everything a little differently
  • If codes are present, gather data about the codes
    • What is the code enabling criteria?
    • Is a stored code suspending some onboard tests until the stored code is cleared?
      • You may fix a problem that allows suspended tests to start running again
      • A suspended test could then set another (different) DTC and the customer thinks it is all the same.

Possible Pattern Failure?

  • Many shop information systems report tech supplied fixes that would help to identify a pattern failure, such as:
  • Look at these diagnostic databases to see if the problem is common.
  • Use this to help put the focus on a system, but still test!

Step 4 – Perform Single System Diagnostics

  • At this point, the problem should be narrowed down to a single system.
  • Run modern/efficient diagnostic tests that eliminate as many potential problems from the system as possible.
  • Work down to pinpoint tests to find the bad component.

Step 5 – Perform & Verify the Repair

  • After approval, perform the repair
  • Verify the repair by running the same tests that identified the original problem
  • If at all possible, clear all
    • Then make sure all monitors can run to completion without setting a hard or pending code before returning the car back to the customer.