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The four most common faults in used cars

Thinking of buying a used car? Our in-depth survey of thousands of car owners reveals the most common faults you need to look out for

The four most common faults in used cars

If you’re buying a used car, it’s crucial you choose one that won’t let you down, particularly if it’s no longer likely to be covered by the original manufacturer warranty. Thanks to our extensive survey of car owners, we know the most common faults you need to look out for.

Our research has also discovered that cars don’t age at the same rate, and brands certainly aren’t equal when it comes to their long-term dependability.

Plus we’ve uncovered a difference amongst fuel types. Based on the results of the latest Which? Car Survey, which includes detailed ownership feedback from 47,013 drivers, covering 55,833 separate models, petrol hybrid cars are proving the most resilient. This is despite the complexity of combining the dual efforts of both the petrol and electric motors.

So to minimise the potential for problems in a used car, buy a hybrid model from a manufacturer we rate highly for reliability.

No car is immune from faults, however. Below are the most common faults that raise their head for drivers of older cars, across all brands and models, and how you can spot them.


Want to know which manufacturers make cars you can trust? We reveal the most reliable car brands.


Second-hand electric cars: what you need to know

While electric cars haven’t been on the market long enough for us to have as detailed reliability data as for petrol and diesel models, we do have data on a range of electric cars registered between 2014-2020.

This tells us that 97% of EV owners haven’t needed to get a new battery – at least allaying some fears over the dependability of expensive lithium-ion battery packs.

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1. Suspension and used cars

Issues with suspension components, including faulty or broken springs and dampers, are the most common cause of headaches in cars over nine years old. Our research shows that it affects a staggering one in ten drivers of cars of this age.

Don’t think you can check the state of a car’s suspension by jiggling in the seat, or giving the body a wobble – it’ll reveal very little, if anything.

The best way to check for potential suspension problems – aside from a professional inspection – is to take it on a drive on a variety of roads. This includes over speed bumps and rough surfaces.

It’s likely the suspension needs some attention if you experience any of these:

  • You can hear any thumps or knocking as the car goes over bumps
  • The handling feels exceptionally baggy
  • You’re suffering jarring jolts in the cabin.

Sometimes the inevitable will happen and even the most reliable car can let you down. Avoid being stuck at the roadside for longer than necessary – we reveal the best car breakdown cover providers.

2. Flat batteries

Battery problems are pretty inevitable as a car ages, but it’s fairly easy to catch the issue early before being left with a car that won’t start.

This is particularly important during the current winter lockdown, as inactivity and cold weather are both significant factors that affect that amount of charge in your battery.

The easiest way to check a car’s battery is in good health is simply to start the car. If the engine struggles to start, the starter motor isn’t working as hard as usual, or the interior or exterior lights dim significantly during ignition, it could mean there’s a battery problem.

To maintain the charge in your battery, you should aim to keep the car running for at least 10-15 minutes continuously each week. Try to avoid stopping and re-starting the engine again in quick succession. This will require even more power, draining the battery further.

If your battery is too flat to turn the engine over, you may need to jump start it – watch our video and find out how to jump start your car safely. You may even replace the battery altogether, depending on its age and condition.

The battery will be checked as part of your car’s MOT, but some garages around the UK offer free battery checks, if you don’t want to wait.

Find a mechanic you can trust – go to how to find a good garage.

3. Exhaust & emissions

The third most common problem affecting owners of older models relates to exhaust components and emissions. Often, visible smoke from the exhaust pipe of a car that otherwise seems to be running fine can be a warning sign of underlying issues.

Blue and black coloured smoke are of particular concern:

  • Blue smoke suggests your car is burning engine oil, pointing towards failing seals and gaskets
  • Black smoke is a result of a number of different issues, but ultimately means your car isn’t burning fuel at the correct rate.

Exhaust/emission issues might also be diagnosed by listening to the engine as it starts up – is there excessive or unusual noise? If so, this could be a sign of a ‘blowing’ exhaust, which is a result of corrosion causing small holes in the pipe.

When you’re checking out a used car, ensure it’s cold when you first start it. Engines run more cleanly and smoothly when warm. If the gauge reads warm just after starting it, the seller may have pre-warmed it and could be trying to hide a problem.

From climate changing CO2 to toxic NO2 emissions, make sure any car you’re considering doesn’t have high emissions – use our free low emission car checker.

4. Air conditioning and used cars

Air conditioning needs regular use and occasional maintenance to ensure it remains effective and clean over time.

Typically, loss of cooling can be remedied by a ‘re-gas’, which will refresh the coolants required by the system. If the air-con isn’t blowing even remotely cold, it could be a fault with the air conditioning unit itself – which is far more expensive to replace.

Regular use is the key to keeping your air con fault-free. Not only will it keep the seals in good condition, it minimises the chance of mould and other bacteria developing in your ventilation system. If your car whiffs of cheese or old socks when you’ve got the air-con on, this is the most likely cause.

Air-con isn’t part of a car’s MOT and air-con recharging/re-gassing might not always be included in a service, so it’s up to you to get it checked and maintained regularly.

Want to know which new or used cars won’t let you down? We reveal the most reliable cars.

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