A full 57% of used car owners have not invested in an extended warranty for their vehicle, according to an exclusive whatcar.com poll.
On top of that, only a third of warranty holders had researched the market themselves to find the right policy for their needs.
The remaining two thirds could suddenly find that a costly failure on their car isnt covered by their policy.
What Car? editor-in-chief Chas Hallett said: 'The average cost of a visit to the garage for used car owners is approximately 350, which is similar to the cost of an extended warranty.
'Thats why wed urge everyone to get a warranty included in any used car deal, to future-proof themselves against any nasty surprises.'
However, What Car? also warns that not all extended warranties offer the same protection, so its vital that people read policy small print carefully before signing up.
In general, used car warranties do not cover wear and tear claims, including tyres, bulbs, wiper blades, brake pads, water and oil pumps, drive belts, electric window motors and fuel injectors.
Chas Hallett said: 'The onus is very much on the buyer to study the policy small print before purchasing a warranty. In general, extended warranties can save the motorist a lot of money if the car has a problem, but only if the right warranty has been selected.
'We advise buyers not to accept the first warranty the car dealer offers and not to select on price alone.'
What Car? top 10 checklist for choosing a warranty
1) Can you choose different levels of cover?
2) How does the age and mileage of the car affect the cost?
3) Is wear and tear included?
4) Which parts are designated as wear and tear?
5) Are failures at an MoT test and service covered?
6) Are extras such as accommodation, car hire and emergency travel included?
7) What percentage of parts and labour are paid for?
9) Is consequential damage covered?
9) Can you transfer your policy with the car to its next owner?
10) Do betterment charges apply?