Our younger readers won’t remember the time when a mechanic used to lift the bonnet of a car, suck air through their teeth. Then they’d start poking around to find where the problem with stiff steering, a lack of acceleration, or one that kept stalling, came from.

Nowadays, finding the root of a problem is much simpler. Car engines are becoming increasingly complex, governed and controlled by a team of computers and monitored by hundreds of sensors throughout each vehicle.

Your ECU and the car engine diagnostic test

Every car rolling off a production line today will have an ECU. This is your car’s brain—an on-board computer known as the Engine Control Unit. Your ECU is where each of your engine’s sensors reports anything amiss, and if there’s a problem, it will illuminate an appropriate warning light on your dashboard.

Your ECU monitors your major engine functions, electrical components, and measures your vehicle emissions. When a sensor delivers a fault, the code is stored in your car’s computer system. Trained technicians and mechanics can then access the fault codes to determine the component that isn’t operating correctly. Once repaired, the mechanic can also clear any fault codes and warning lights.

So, what happens with that warning light when you take your car to a garage?

The car diagnostic check and your on-board diagnosis

A car diagnostic test will deliver details from your car’s many sensors to alert your technician to which component isn’t working correctly. Armed with this new information, your mechanic, with their wealth of experience and knowledge, will perform a deep-dive into locating the real problem.

The codes won’t tell the mechanic how to repair the car, only offer which components have triggered a fault. Think of these codes as symptoms that the mechanic can utilise to find out what the real problem is.

Driving the car, you could also be aware of symptoms, for example, poor acceleration, stiff suspension, heavy steering, and any other anomalies in your car’s performance.

Your car’s on-board diagnostic system—or OBD for short— monitors everything that goes on in your ECU and delivers fault or trouble codes via a plug-in device.

System diagnostic tools and systems

Mechanics have special state-of-the-art tools that plug into your car that read the fault or error codes.

They come with a few different names, but they each perform, pretty much, the same job.


EOBD is the abbreviation for European On-Board Diagnostics.

The EOBD system was introduced in 2001 for petrol cars in Europe (2003 for diesel models) to measure engine emissions. Each car has a universal socket that allows a scanning tool to retrieve fault codes and readings.


EOBD2 is not an updated system but a second-generation EOBD that has access to manufacturer-specific features available on more advanced EOBD tools.


The original OBD system was in operation pre-1996 in the United States. The US government introduced legal standards in 1996 for all manufacturers to follow; this standard was OBD II.

Despite it being an American format, most tools for sale in the UK go by the same name. They do the same job, delivering similar codes, yet a few other names have been thrown into the mix to help complicate matters for everyday drivers.

What issues can a car diagnostic service detect?

  • Ignition timing issues
  • Build-up in the engine
  • Fuel injector performance
  • Whether coils are firing
  • RPM levels
  • Air and coolant temperature
  • Crankshaft and camshaft positions and throttle openings
  • Battery warnings
  • Bulb failures
  • Steering problems
  • Overheating
  • Failure to start
  • Gear selection and transmission
  • Overheating and smoking

These are only a selection. Your ECU monitors so much that goes on under your bonnet and throughout your vehicle. If there’s a problem with your car, it’s highly likely that your EOBD will have spotted it.

Common car diagnostic codes

Despite the equipment, plugs and sockets all following industry standards, the codes themselves are almost always specific to each manufacturer.

The rule of thumb is that fault codes follow a format of letters and numbers. The majority of codes will range from P0100 to P1899. The P usually refers to Power Train, and other letters, B for Body and C for Chassis, for example, refer the technician to specific areas of the vehicle.

As mentioned, each manufacture has a unique database of codes referring to each fault. So, once the fault code is found, the database for that make and model is consulted to find out what the error code means.

What is the average cost of a car diagnostic test?

A typical diagnostic test costs around £50. There will be a little variation depending on your make and model and who carries out the work, but you can expect to pay somewhere between £40 and £65.

Fixter can help find a car diagnostic near you and save you money. In some cases, we save our customers up to 30%. Drop us a line or give us a call to see how much we can help you.

Top 10 car diagnostic tools in the UK

According to bestreviewer.co.uk, the top 10 car diagnostic tools in 2020 (so far) are:

  1. U480 OBD2
    Reads and erases trouble codes with access to manufacturer specific readings.
  2. Ancel AD310 Classic
    The AD310 has a large 128x64px LCD backlit display, contrast adjustment and a 2.5’ long heavy-duty cable.
  3. Autool CS320
    Supports all OBD-II protocols to receive generic and manufacturer specific codes.
  4. Foxwell OBD2 Diagnostic
    Reads domestic and imported vehicles with a unique one-click I/M readiness status.
  5. Intey OBD2 Diagnostic
    Easy to use with a freeze-frame operation—one of the best value models of this selection.
  6. NI100 Diagnostic Scanner
    Detects and turns off all codes, comes with a long cable and provides additional emissions smog testing.
  7. Autel Autolink AL539B
    Neat, compact and comprehensive. A robust tool with a built-in multi-meter.
  8. Foxwell Professional OBD2 Scanner
    Designed for professional or DIY use, this model has a powerful chip to provide a super-fast diagnosis, and in several different languages.
  9. Autel Diaglink
    As well as the usual engine fault code functions, this model checks airbag operation, gearbox status, ABS circuits and much more besides. It’s one of the pricier models, but it does offer an awful lot more bang for your buck.
  10. Launch CRP123 Reader
    The most capable of the 10, but also the most expensive. As well as checking all the usual codes and the more in-depth diagnosis options, this model works in various languages. It’s also compatible with Windows operating systems for additional downloads.

Vehicle diagnostic testing—saving time and money

If you’re looking into efficient ways of ‘how to fix my car’, you could save a great deal of time troubleshooting and testing by using the right diagnostic scanning tool.

Although the tool will never pinpoint what repair you need to carry out, the information they do supply can go a long way to narrow down your search.