Explaining the Diagnostic Process

Airdrie Automotive Diagnostics

Written by Chris Dekker, former co-owner at Tools in Motion.


The other day, a customer called us to ask if we had “one of those computers that tells you what’s wrong with the cars”. I was tempted to make a goofy response like “Yes, and I have robots that fix them for me, too!”, but I knew what they meant. The customer was talking about a scan tool. And of course, we do have scan tools; four different ones, in fact.

Scan tools have been around for decades and became a very necessary tool in the 1980s as vehicles switched from carburetors to fuel injection, which of course meant they now had an on-board engine computer. Today’s vehicles have between 5 and 50 on-board computers; the scan tool is a device that lets a qualified technician communicate with these on-board computers in order to diagnose problems, test systems, and make software changes. Today’s scan tools – many of which aren’t actually “tools” anymore, but laptop-based programs – are pretty advanced. That being said, most customers don’t realize how little the scan tool actually contributes to an accurate diagnosis, and how quickly in the process the human brain must take over from the tool.

One of the more regular things that we use a scan tool for is to diagnose a warning light, such as a Check Engine or ABS light. Whenever a warning light comes on, this means that an on-board computer has detected some sort of problem, and stored a “trouble code” as a result.  In the case of the Check Engine light, these codes read something like P0302 or P0171. Any code starting with P0- is universal between most vehicles, so the scan tool will usually add a definition so they read P0302 – Cylinder #2 Misfire and P0171 – Fuel System Lean, Bank 1.  Retrieving these trouble codes from the vehicle is usually a quick process, and this is where two common customer misconceptions (and sometimes, sources of frustration) come from:

  1. Because the code retrieval process (which is only the start of the diagnosis, as we’ll get to in point #2) is so quick, customers sometimes feel ripped off when they are charged $100-150 for this service. This is partially our fault, as most shops will show the service as a one hour charge on the invoice. If the service doesn’t take this long, it’s only natural for customers to get upset. It’s very important for us to explain that we’re billing customers for a $120 charge vs one hour’s labour. This might be the same dollar amount, but we need to justify the charge for what it really is: in part, a way to earn back the purchase cost of the scan tools being used. In total, our four scan tools cost over $25,000 and require another $3500 in software updates every year.
  2. Many customers also don’t realize that retrieving a trouble code is only part of properly diagnosing an issue. This is partially because we as professionals haven’t done a good enough job educating consumers; parts stores that will “pull a code for free and then sell you a part” aren’t helping the situation. For example, what’s causing that P0302 – Cylinder #2 Misfire? Is is a spark plug? A plug wire? It could also be an ignition coil, a fuel injector, a wiring problem, a bad sensor, a vacuum leak – even a mechanical condition such as low compression on that cylinder, a sticking valve, or broken valve spring. This is where a properly trained technician – and a whole bunch more test equipment – are required to narrow in on the real problem.


Here are some of the steps a properly trained technician will take in order to diagnose most Check Engine lights, once the easy part (pulling the code) is complete:

  • Using the scan tool, monitoring the PIDs (or parameter IDs) for the affected module. These are hundreds of numbers that the scan tool spits out in real time, constantly updating them many times per second. These can be temperature readings, voltages, duty cycle (on/off time) numbers, resistance values and more; and they have confusing names like B1S2 Ho2S, VGT DUTY % and VREF, etc:
    Airdrie Auto Diagnostic Numbers

    Some basic engine PIDs being graphed

    None of these numbers really tell you anything by themselves; it takes a skilled technician to know which ones are relevant to the problem being experienced, and to understand the relationship between these numbers. (When number A increases, number B should increase at the same rate, while number C decreases at half that rate, etc.) Interpreting all of these numbers will help the technician understand what’s going on inside the engine, and narrow on on which part of the engine is having trouble.

  • The technician will likely also consult published service information from the vehicle’s manufacturer – which we subscribe to – in order to familiarize themselves with how certain systems work, what readings to expect, etc. It’s impossible to remember everything about every single vehicle on the road, which makes the tech’s ability to understand service information, diagrams and wiring schematics very important. These service information subscriptions aren’t cheap, either – but like the scan tools, are a very necessary part of a shop’s tools and equipment budget.
  • The tech will probably also check for Technical Service Bulletins (TSBs). These bulletins, released from the auto manufacturers to service facilities like ours, serve to alert us to common issues we should know about. They may also contain information on updated test procedures, updated fluid recommendations, new specialty tools that have been released for a given repair, revised/improved parts that are available to fix certain issues, and more. A quick check of the TSBs will help the technician ensure they aren’t missing any important information that they need to diagnose an issue accurately and carry out a proper repair that lasts as long as possible.

At this point, the technician has probably exhausted all of the information that the scan tool, and their computer, can give them. Finalizing the diagnosis will probably require some hands-on testing with a variety of other tools. Sticking with the P0302 – Cylinder #2 Misfire, the tech may use:

  • An ohmmeter and spark tester to check the spark plug wires and ignition coil.
  • A fuel pressure tester and injector pulser/balance tester, used together to test the fuel injector flow rates.
  • A multimeter, test light, noid light, oscilloscope or other tools to check for wiring issues associated with the sensors, ignition coils and fuel injectors.
  • A vacuum pump, vacuum gauge or smoke machine to rule out the possibility of vacuum leaks.
  • An exhaust backpressure gauge, or pressure transducer to check the engine’s air pumping action and rule out a restricted exhaust system.
  • A compression tester, cylinder leakage tester or borescope to check for internal, mechanical engine problems.

Depending on the situation, there may be more – or less –  tests and tools required, of course. We feel that all of the additional work that comes after “pulling the code” is part of the proper, complete diagnosis, and it’s included whenever you pay us to diagnose a warning light on your vehicle. We hope this insight into what we do every day will help our customers recognize the value in the diagnostic charges that they are billed for. After all, by the time we’re done, we should be able to tell you exactly what your vehicle needs without any guesswork. It’s what we’re known for, and our reputation depends on it!

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