Vehicle Diagnostic Techniques (2024)

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If a vehicle system isn't operating correctly, then it should be repaired. This is where diagnostic and other skills come into play. It's important to recognize that something isn't operating the way it should be by applying your knowledge of the vehicle system, then applying this knowledge further and combining it with the skills of diagnostics to be able to find out the reason.

Finding the problem when complex automotive systems go wrong is easier if you have the proper knowledge. This knowledge consists of two parts:

An understanding of the system in which the problem exists.
The ability to apply a logical diagnostic routine.

It's also important to know about these definitions:

Symptom – What the operator of the vehicle notices.
Fault – The error in the system that result in the symptom.
Root Cause – The cause of the fault.

Note: General diagnostic principles and techniques can be applied to any system, physical or otherwise.

The Six-Stage Process

Verify: Is there actually a problem? Can the symptoms be confirmed?
Collect: Get further information about the problem through observation and research
Evaluate: Stop and think about the evidence
Test: Carry out further tests in a logical sequence
Rectify: Fix the problem
Check: Make sure all systems now work correctly

Note: The six-stage diagnostic process is recommended but there are others that are similar.

After using the Innova 5610 to read the trouble codes, the mechanic goes through the first 4 of the 6 steps in the Six-Stage Process. He knows how to rectify the issue now but needs to fix the problem, then check to make sure all systems work correctly.

Example 1:

Here is a very simple example to illustrate the diagnostic process. The reported fault is excessive use of engine oil.

Find out how much oil is being used.
Examine the vehicle for oil leaks and blue smoke from the exhaust. Are there any service bulletins?
If leaks are found, the engine could still be burning oil but leaks would be a likely cause.
A compression test, if the results were acceptable, would indicate a leak to be the most likely fault. Clean down the engine and run it for a while. The leak will show up better.
Change a gasket or seal, etc.
Run through an inspection of the vehicle systems particularly associated with the engine. Double-check that the fault has been rectified and that you have not caused any further problems.

Example 2:

Here is another example using a vehicle's cooling system that has a fault. Remember that the diagnostic procedure can be applied to any problem – mechanical, electrical, and even medical. In this case, let us assume that the reported fault with the vehicle is overheating. As it's quite common in many workshop situations, this is all the information we have to start with. Go through the six-stage process.

Stage 1 – Take a quick look to check for obvious problems such as leaks, broken drive belts or lack of coolant. Run the vehicle and confirm that the fault exists. It could be the temperature gauge, for example.
Stage 2 – Is the driver available to give more information? For example, does the engine overheat all the time or just when working hard? Check records, if available, of previous work done to the vehicle.
Stage 3 – Consider what you now know. Does this allow you to narrow down what the cause of the fault could be? For example, if the vehicle overheats all the time and it had recently had a new cylinder head gasket fitted, would you be suspicious about this? Do not let two and two make five, but do let it act as a pointer. Remember that in the science of logical diagnostics, two and two always makes four. However, until you know this for certain then play the best odds to narrow down the fault.
Stage 4 – The further tests carried out would now be directed by your thinking at stage 3. You do not yet know if the fault is a leaking head gasket, the thermostat stuck closed or some other problem. Playing the odds, a cooling system pressure test would probably be the next test. If the pressure increases when the engine is running, then it is likely to be a head gasket or similar problem. If no pressure increase is noted, then move on to the next test and so on. After each test go back to stage 3 and evaluate what you know, not what you don’t know.
Stage 5 – Let us assume the problem was a thermostat stuck closed – replace it and top up the coolant, etc.
Stage 6 – Check that the system is now working. Also check that you have not caused any further problems such as leaks or loose wires.

Concern, Cause, and Correction (CCC Process)

The three Cs, as concern, cause, and correction are sometimes described, is another reminder that following a process for automotive repairs and diagnostics is essential. It's in a way a simplified version of our six-stage process. So, while the concern, cause, correction sequence is quite simple, it's very effective as a means of communication as well as a diagnosis and repair process.

A. Customer Concern: Battery seems to be discharged and will sometimes not start the car. It seems to be worse when the headlights are used. This should set you thinking that the cause is probably a faulty battery, a charging system fault, a parasitic discharge or a starter motor problem (the symptoms would suggest a charging fault is most likely but keep an open mind).

Vehicle service history information: Car is five years old, has done 95,000 miles but has a good service history. A new battery was fitted one year ago and the cam belt was replaced two years ago. Battery probably OK and drive belt adjustment likely to be correct (still suspicious of a charging fault)
Related technical service bulletins: New camshaft drive belt should be fitted every 50,000 miles. Not connected, but it would be good to recommend that the belt was changed at this time.
Diagnostic procedures performed: Battery voltage and discharge test – OK Drive belt tension – OK (but a bit worn) Alternator charging voltage – 13 V Checked charging circuit for volt drop – OK. 14 V is the expected charging voltage on most systems.

B. Cause: Alternator not producing correct voltage. An auto electrician may be able to repair the alternator but for warranty reasons a new or reconditioned one is often best (particularly at this mileage).

C. Correction: Reconditioned alternator and new drive belt fitted and checked – charging now OK at 14 V. By using this process, the problem was almost diagnosed before doing any tests. Also, remember that following this process will make you confident that you've carried out the correct repair the first time.

Root Cause Analysis

The phrase Root Cause Analysis (RCA) is used to describe a range of problem solving methods aimed at identifying the root causes of problems or events. It helps to reinforce the importance of keeping an open mind when diagnosing faults, and stresses the need to work in a logical and structured way. The root cause of a problem is not always obvious. Here is an example:

Let us assume the symptom was that one rear light on a car didn't work. Using the six-stage process, a connector block was replaced as it had an open circuit fault. The light now works ok but what was missed was that a small leak from the rear screen washer pipe dripped on the connector when the washer was operated. This was the root cause. The practice of RCA is based on the belief that problems are best solved by attempting to address, correct or eliminate the root causes, as opposed to just addressing the faults causing observable symptoms. By dealing with root causes, it is more likely that problems will not reoccur. RCA is best considered to be an iterative process because complete prevention of recurrence by one corrective action is not always realistic.

The following list is a simplified representation of a failure-based RCA process. Note that the key steps are numbers 3 and 4. This is because they direct the corrective action at the true root cause of the problem.

Define the problem.
Gather data and evidence.
Identify the causes and root causes.
Identify corrective action(s).
Implement the root cause correction(s).
Ensure effectiveness

RCA is usually used as a reactive method of identifying causes, revealing problems and solving them and it's done after an event has occurred. However, RCA can be a useful proactive technique because, in some situations, it can be used to forecast or predict probable events.

RCA is not a single defined methodology. There are a number of different ways of doing the analysis. However, several very broadly defined methods can be identified:

Safety-based RCA descends from the fields of accident analysis and occupational safety and health.
Production-based RCA has its origins in the field of quality control for industrial manufacturing.
Process-based RCA is similar to production-based RCA, but has been expanded to include business processes.
Failure-based RCA comes from the practice of failure analysis used in engineering and maintenance.

Example Faults:

A: The brake/stop lights are reported as not operating. On checking it is confirmed that neither of the two bulbs or the row of high-mounted LEDs are operating as the pedal is pressed. All other systems work correctly.

Two bulbs and 12 LEDs blown.
Auxiliary systems relay open circuit.
Brake light switch not closing.

B: An engine fitted with full management system tends to stall when running slowly. It runs well under all other conditions and the reported symptom is found to be intermittent.

Fuel pump output pressure low.
Idle control valve sticking.
Engine speed sensor wire loose.

C: The off side dip beam headlight not operating. This is confirmed on examination and also noted that the off side tail lights do not work.

Two bulbs blown
Main lighting fusible link blown
Short circuit between off side tail and dip beam lights