Thousands of cars face being scrapped as a result of tougher MoT rules that have just come into force to include safety checks on a raft of electronics that have been introduced in recent years.

In many cases the cost of repairing an expensive electrical problem on an older vehicle will be more than the car's value, leaving the owner with little choice but to take it to the scrapyard.

The Department for Transport added the mandatory tests to the existing MoT on New Year's Day to comply with a revised European testing directive (2009/40/EC). The is to harmonise minimum requirements across Europe and make sure the test reflects the electrical/electronic complexity of modern car safety features.

Until now, for example, those with airbag lights that remain on while driving have passed because it's an issue that hasn't formed part of the safety inspection. Now drivers will no longer be able to simply remove the bulb, as it will have to be shown working when you turn on the ignition, and go out once it is started. It is a similar story for non-working speedometers and other electronic features such as stability control systems.

There is a breathing space, however. Vosa, the government agency responsible for the MoT, has indicated that up to "late spring" – probably May, although the exact date hasn't been set yet – "failures" relating to the new tests will first be treated as "advisories". This means that if you are tested now and have a warning light that's faulty you will have until the next MOT renewal date to get it fixed.

The move will mostly affect the owners of older cars, typically those dating back to the late 90s and onwards which feature airbags and the like but are only worth three figures. Owners of some newer cars will face big bills to get their vehicle through the new MoT.

Included in the new check list are:

• Electronic stability controls and antilock brakes.

• Electronic parking brakes.

• Headlight main beam warning light.

• Electronic power steering warning light.

• Brake fluid level warning light.

• Seat belt pre-tensioner warning light.

• That the steering lock functions.

• Power steering fluid level must be above the minimum level indicated on the reservoir.

• The tow bar's electrical socket. There will be a basic security/damage check of seven-pin sockets; 13-pin sockets will be subject to a full electrical connectivity check and incorrectly connected or inoperative circuits will result in failure.

• Visible wiring that is insecure, or likely to cause a short circuit.

• A speedometer is not fitted, is incomplete, inoperative, has a dial glass broken/missing, or cannot be illuminated.

• Testers will check that the driver's seat can be moved.

• A rear door that cannot be opened from the outside will fail.

• The removal of catalytic convertor that was fitted as original equipment.

Vosa chief executive Alastair Peoples says: "The MoT test is designed to make sure that a vehicle is fit to be on the road and so it needs to be updated to reflect new technology.

"We have worked with the industry to prepare them for these changes to make sure that the measures are introduced in the least burdensome way possible. Although these electronics are new to the MoT, it has always been a legal requirement to keep your car serviceable and safe."

A spokeswoman for the AA says the need to have airbag warning lights going out correctly will probably have the biggest impact. "Undoubtedly, these measures will lead to some cars being scrapped where the cost of repairs will outweigh its value."

At present, all cars over three years old have to be tested each year. The AA also says it is "very concerned" about a government proposal to move to two-year testing as many people only get their car checked for an MoT.

If you run a car that is likely to fail the new test later in the year, there is nothing to stop you getting a new MoT now when you will be issued with the advisory rather than failed.

That would allow you to run a car that, say, had a faulty speedo for a further year. But it is not advisable to run a car with a potentially faulty airbag. It should be checked to make sure there is no risk of it going off unexpectedly.

What's the most fuel-efficient ever?

Consumer group Which? has just tested its most fuel-efficient car ever. The new Toyota Prius plug-in hybrid, which has a conventional petrol engine, and electric motor combined, managed a remarkable 235.4mpg in a test of stop-start city driving.

It even managed to break the elusive 100mpg barrier on its extra urban cycle. Overall it recorded a combined fuel consumption of 83.1mpg.

Which? said the car was let down by a "less remarkable" 46.3mpg at motorway speeds. The new Prius beats the previous fuel economy record set by he VW Polo 1.2TDi Bluemotion, which averaged 74.3mpg in Which? Car tests.

Unlike the existing Prius Hybrid, whose electric motor is recharged by the onboard petrol engine, the plug-in hybrid's battery can be fully recharged from a domestic electricity socket in 90 minutes costing around 64p at current rates.

This gives it a much longer range using electric power only (up to around 14 miles.)

The Prius Plug-In Hybrid is due to go on sale in the UK in the summer of 2012, for £31,000. But it qualifies for the government's electric car grant scheme, so that's reduced to £26,000.

Which? says the "retail" version that arrives next year will have a lighter, more compact battery pack and this raises the prospect of enjoying even better fuel economy.