How it works: diagnostic scan tool

Actron AutoScanner auto-diagnostic scanning device.
Date:27 June 2014 Tags:, , , , , , , ,

The scan tool is the first step in diagnosing modern automotive troubles. Here’s how it works. By Mike Austin

Today’s cars are so complicated that it might seem like any kind of repair requires a degree in electrical engineering. That’s not quite the case.

True, our cars are electronically complex, but built-in diagnostics come as part of the deal. Whereas old cars required special tools and a lot of guesswork to identify an engine problem, today’s passenger vehicles can tell you everything about the way they are running, thanks to OBD-II ports.

OBD stands for On-Board Diagnostics. This functionality was a response to a mandate by the California Air Resources Board that required all cars from 1991 onwards to be able to monitor emissions-related systems. The second generation (OBD-II) was implemented five years later and established a standard connector and communication protocol for all car manufacturers. The port is always located in the driver’s side footwell, under the dash. The standardisation of this port means that any tool can be used to look at data from any car.

Beyond the basic codes, some carmakers have manufacturer-specific data that offer even more information. And because the OBD-II can also report data such as vehicle speed, throttle position and brake position, it’s handy for more than just troubleshooting. Insurance companies, fleet managers and parents of teen drivers also use it to log data about driving habits.

The basics
Say you get a check-engine light. (That worrisome warning is called the malfunction-indicator lamp, or MIL, because it can signal a fault in any number of your car’s systems.) To find out what’s wrong, attach an OBD-II tool to the car’s port, start the car, and activate the scanning function. The tool will come up with a list of active fault codes and what they represent, such as “P0300: Cylinder Misfire Detected.” (If your scanner shows only the fault code, you can look up the explanation online.) You now know where to begin troubleshooting. In this example, the problem is probably related to the ignition system. You can clear the MIL with the scan tool, but we don’t recommend doing that until you’ve solved the problem.

Going deeper

There are more than 3 000 generic fault codes, in addition to manufacturer-specific codes. Not all of them point to a clear solution. But chances are that someone else has already solved this problem and shared the story on a message board – just do an online search for your car to find an existing community. If you ­ x a lot of cars or have a high-maintenance vehicle, there are also subscription based services such as Actron’s that point you to model specific repairs for each code.

Sometimes a fault is tough to trace down, or you simply need more information to decide how to proceed. More sophisticated tools show you the state of your car’s sensors at the time of the fault or display live data. Those extra clues help determine exactly when a fault is happening and what else is going on with the car, but basic automotive problem-solving is also essential here. You want to avoid replacing every part that could be malfunctioning and isolate the possible causes. If the engine is running rich, for example, you can check the signals coming from the intake airflow and oxygen sensors and compare them with the expected values (found online or in a service manual).

If you end up stumped, or a repair is simply too big to take on, the OBD-II tool is still helpful when you take a car to a service centre. A good mechanic, like a doctor, will listen to specific symptoms as a starting point for a diagnosis.

Three more auto diagnostic aids, each starting under R500:
Sensors on your car convert temperature and other readings to voltages that the car’s computer can understand. This tool lets you check a sensor’s output voltage to verify if faulty wiring or a bad part is creating out-of-spec signals that might be the cause of your troubles.

Vacuum-pressure gauge
When a car’s engine is running poorly, it’s usually related to air, fuel, or spark. This tool covers the air department, so you can work through the maze of vacuum lines under-bonnet to track down a leak or cracked tube that could be throwing things off.

Fuel-pressure gauge
The vacuum-pressure gauge can check fuel pressure on a carburetted engine, but modern fuel-injection systems require a dedicated fuel-pressure gauge. And knowing whether the engine is getting enough – or too much – fuel is one of the first steps in automotive troubleshooting.

  • Jim Campbell

    I have a problem with a Peugeot 407 2 litre diesel that will not start, though the obvious possibilities are not the problem, as the engine turns over, and there is smoke issuing from the exhaust pipe.

    I recently purchased two versions of an ELM 327 code reader with software for OBD-II codes, and also a number of Android based diagnostic programmes, none of which are giving sufficient/suitable information to resolve the problem with the car. I can only get one reading of a “glow plug” problem with all of them and nothing else.

    I understand that Peugeot vehicles are notorious in that only the Peugeot workshop diagnostic equipment (not available to the public) is able to get into the vehicle diagnostics to determine faults, and consequently one is tied in to an expensive diagnosis and repairs.

    • tytower

      All diesels benefit from some sort of heater inside the cylinder head. That helps the diesel ignite when it is compressed . Fix the glow plugs and it will probably start