How to diagnose common car faults

A warning light can spell danger, but figuring out what’s causing the fault can frustrate amateurs and professionals. In this guide, we explain how to diagnose common car faults and how to use a diagnostic code reader.

Common car faults (and how to diagnose them)

1. Warning lights: The warning lights on your car provide critical information on the overall health of the engine, fluids, brakes and other vital systems. The symbols used are quite universal and should be easy to understand (if you’re unsure what the symbols mean, check out your car’s handbook or read our guide to car warning lights if you need a reminder).

Warning lights are colour-coded generally with green for awareness, amber for monitoring and red for urgent action. An amber warning light means your car has developed a problem and you need to investigate. Some issues will be obvious; others may need a fault code reader or an expert diagnosis at a Halfords Autocentre. Red lights are critical warnings that could be dangerous, and you should pull off the road immediately and safely to check out the fault.

Intermittent warning lights can be frustrating for amateurs and professionals. Our advice is to monitor the situation and use a warning code fault reader to understand and fix the problem. Be aware that some warning lights can be an MOT failure, so diagnosing and fixing problems is vital.

2. Squealing brakes: Squeaking and squealing brakes indicate something could be wrong and needs attention. While the weather can cause brake noise, if it persists, it's highly likely that your brake pads could be worn and need to be replaced. This is a job a confident DIY mechanic can do at home by following our guide on how to change your brake pads.

3. Tyre pressure problems: Regularly check your tyres for wear and damage and ensure they're inflated to the correct pressure. If your tyres are losing pressure quickly, then you’ll need to investigate. You can visually inspect your tyre for wear, damage and anything that may be sticking out of it like a screw or a nail and causing it to leak air. If you can't spot any obvious reasons why it's losing pressure, book a free tyre check with Halfords.

4. Leaking fluids (oil, coolant, water): If you spot fluids underneath your car, the first step is to identify the liquid. Oil is black and sticky, coolant can be orange or red, petrol and diesel is iridescent, and water is transparent. If you spot water under your car, it could be due to be condensation from the air conditioning system. Oil and coolant under the vehicle suggest a leak somewhere in the system, so you'll need to investigate.

Common causes of fluid and oil leaks include split hoses, worn seals and gaskets. The location of the fluid loss (like a puddle on the floor) should provide some clues as to its cause, but you'll likely have to do some investigation. Lifting the bonnet will give you access to the engine bay, but you may also need to raise the car up to gain access underneath. Any leaks should be investigated thoroughly and fixed. If you don't feel confident doing so, book into a Halfords Autocentre.

5. Flat battery: Car batteries can lose power over time before they fail, potentially leaving you stuck and stranded. Some warning signs that your battery is losing power are difficulties starting the engine, dim headlights. or electrical issues. You can check the physical condition of your battery by lifting the bonnet and looking for corrosion on the connectors, a damaged or bulging battery case or a fluid leak. If the problem persists, book a free Halfords Battery Health Check, where we’ll look at the charging system and provide impartial advice on how to fix it.

How to diagnose common car faults using a diagnostic reader

Sometimes, the issue with your car can be challenging to diagnose and require a more detailed investigation using specialist tools. Diagnostic code readers communicate with the computer inside your car, providing information on faults.

When plugged into your car, the diagnostic reader will show codes that provide information on the area that the fault is located. This helps direct mechanics to what is causing the problem with your car and identify a solution.

Here's how to identify common faults in your car with a diagnostic reader:

How to use a diagnostic reader

  • Locate the OBD port: Find your car's On-Board Diagnostic (OBD) port. The location varies by vehicle, so check your owner's manual or search online to find where it is in your car. Common spots include under the dashboard, near the steering column, or close to the centre console.
  • Connect the diagnostic code reader: Once you've located the OBD port, plug in your diagnostic code reader. This device communicates with your car's Engine Control Unit(s) (ECU) to provide information on faults.

Interpreting diagnostic codes

  • Read the fault code: If there's a problem with your car, the diagnostic code reader will display a code (or series of codes if there is more than one problem). Write the codes down on a piece of paper or put them in your phone.
  • Check the code: You can find out what the codes mean by checking the manual that came with your diagnostic code reader.
  • Common faults: Your diagnostic code reader should point you towards the area that is causing the problem with your car. If you see a code starting with "P01", "P02", and "P03", these are often related to the fuel system, a common cause of engine problems.

Troubleshooting common fuel system faults

Here are some of the standard engine fault codes and what they mean.

  • P01 faults: This error code indicates a problem in the circuit between the ECU and the fuel pressure regulator on the injection rail. This can often cause the car to enter “limp home” mode. Causes could include a disconnected fuel volume regulator plug, a damaged fuel pump or corrosion on the sensor connector.
  • P02 faults: This error code relates to an electrical anomaly in the fuel system higher than normal. Again, like fault code P01, this can be caused by a disconnected fuel volume regulator plug or a damaged fuel pump.
  • P03 faults: Like the above fault codes, P03 faults are generated when an electrical reading in the fuel system is below normal. The causes are a damaged fuel pump or sensor.

Next steps

You’ve identified the fault codes and likely causes, now it’s time to fix them.

  • Note the fault codes. These will be provided by the diagnostic code reader.
  • Check. Begin by checking the electrical and mechanical connections in the areas that the diagnostic code reader has pointed you to.
  • Clear and recheck. After checking the areas suggested, clear the fault codes, and drive the car for a short period. If the fault with the car or warning light flashes again, it’s a persistent issue that needs to be fixed.
  • Further inspection. Check the fuel and air systems for any leaks which could be the cause of the issue. You may need to raise the car up to do this to gain access to the bottom.
  • Consider replacing parts. If the troubleshooting process doesn't work, then consider changing or replacing parts to see if this fixes the problem but be aware this could prove costly if you cannot pinpoint the exact fault. For step-by-step instructions on fault finding and how to replace parts, check your car’s workshop manual. You'll find specific information and advice that's relevant to your vehicle.

Finding and fixing common car faults

By following these steps and using a diagnostic code reader, even those with basic DIY skills can begin to understand and address common issues. Our advice is to never ignore a warning light or any other sign that your car is in trouble, as it’s only going to get worse.

If you're unsure of the cause of a fault or need your car fixed, book into a local Halfords Autocentre. Our expert mechanics are always on hand to ensure your car is safe and road ready.