Diagnosing Fault Codes
Monday, 05 October 2009 12:11
Processes and Information to Use When Diagnosing Fault Codes

Trouble codes are an invaluable aid and starting point when diagnosing faults, and they are often the only thing that shows up when there is a problem with a vehicle.

By using a process and checking other diagnostic information available, you can perform more accurate fault finding, saving time and money.

Ideally every mechanic wants a tool that they can plug into a car which will find the exact location of the fault and tell them what to do to fix it, unfortunately this is still science fiction (at the moment).

To start with download this handy check list from the SP Diagnostics Website, it will help you in recording vital information, prompting you to ask the customer key questions about the fault and make your diagnostics process easier.

Free Workshop Diagnostics Check List

You will end up providing a more professional service and have a more comfortable relationship with the customer when charging for diagnostic tests.

Start to fill out the form

When the customer comes in, spend a few minutes filling the start of the form out, what lights were on, when it happens, under what conditions, is it still present.. etc.. etc..


Next hook up the SPi or ACR4 diagnostic equipment and read the fault codes, either using the manufacturer specific software or via EOBD.

Remember that the ECU produces codes based on the symptoms it sees, which may not necessarily be the true cause of the fault.

Once you have a code or codes you can take a look at more information. Often freeze frame data is provided along with the fault code, this gives you the component or live data at the time the fault occurred, this can help if the fault is no longer present and has 'fixed' itself again.

Make sure you write all this information down on the check list, to refer to in the future.

Next take a look at the live component data, for the related systems and components to do with the fault code, compare this against manufacturer's data / your knowledge / similar components on other vehicles etc.. Try and see if anything looks odd or out of place, remember that this data can vary depending on things like engine temperature, engine load, throttle position, so take this into account. You should also check the data that the ECU is providing with a real measurement device like a multimeter, if they don't tie up there could be an issue there with the wiring or connections.

If a fault says there is a short circuit to ground or something similar, check to see if this is true with a meter and see if the voltage on the component's signal pin is low. What happens when you unplug the component? Does it still stay low? Does the fault code change? This will indicate where the fault could be, which side of the connector it is (ECU & Wiring or Component).

If the faults are to do with a component that gets driven e.g. idle air control valve, exhaust recirculation valve etc, you can run actuator tests and measure what the control pin of the actuator is doing. If it is ok then the ECU and drive circuits are probably ok and the fault could be in the component or in its feedback circuit.

After you identify/fix the fault you can clear the fault codes. Re-read the codes after taking the car for a test drive (some trouble codes need the car to be driven for a period of time to either clear or to come back). If the codes have cleared then make a note of this and your job should be done, if they have come back start your process again and check for related components that might be causing the fault. (Poor earths can throw up multiple faults as it causes chaos in ECUs)

By using this or a similar process and recording what you come across, not only will you learn from and be able to refer to previous check lists, you will split the tasks down into more manageable sections.

  1. Start the check list with the customer
  2. Read the error codes
  3. View freeze frame & component data
  4. Verify component readings
  5. Check drive signals
  6. Identify / fix fault
  7. Clear the trouble codes
  8. Take the car for a test drive
  9. Check the codes again

You do not have to carry out all of these steps, but they are available if you need to use them.


The code reader can only display what an ECU is telling it, try and think why the ECU would be getting information that makes it think there is a fault, and check these out.

Sometimes you need to think 'outside of the box' if the ECU is saying there is a lambda sensor fault but it looks fine, check the catalyst out with an exhaust gas analyser, automatic transmission faults might simply mean there isn't enough gearbox oil, and check things like fuses out.

Some faults may not produce a trouble code, imagine a faulty coolant temperature sensor, that always reads 20 degrees lower or higher than it should, the ECU won't be able to tell if this is wrong, as it thinks it's getting a correct temperature reading. This may cause other faults or issues, if the coolant reads lower than it should the engine will be richer for longer during warm up, and may cause issues with plug fouling and emissions systems. So also lookout for fault codes that don't appear to be related to the actual fault.



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