Car Alternator Guide: Failure Signs, Repair Costs and Buying a Replacement

Quick answer

A failing car alternator typically shows warning lights on the dashboard, dim or flickering headlights, electrical accessories that slow down or cut out, and a battery that keeps going flat even after jump-starts. Replacement costs in the UK usually fall between £250 and £800 depending on the make and model, with the alternator itself around £300 and labour adding £150 to £200. When buying a replacement, match the exact make, model, year and engine size, check the amperage rating, and decide between new, refurbished or reconditioned units based on your budget and how long you plan to keep the car.

Common signs your alternator is failing

The alternator charges the battery and powers electrical systems while the engine runs. When it starts to fail, you will usually notice several symptoms appearing together rather than in isolation.

Dashboard warning light
Many cars show a battery-shaped charging warning light, often red; consult the handbook for your vehicle. This does not always mean the battery itself is faulty. It signals that the charging system is not working correctly, and the alternator is the usual suspect.

Dim, flickering or overly bright lights
Headlights and interior lights may appear dimmer than normal, flicker at idle, or occasionally surge brighter. This happens because the alternator cannot maintain a steady voltage supply.

Electrical accessories behaving oddly
Power windows may move more slowly, the radio cuts out unexpectedly, seat heaters stop warming properly, or the air conditioning blower weakens. These systems draw significant current and are often the first to show problems.

Difficulty starting or frequent stalling
The car may crank weakly, stall while driving, or die shortly after a jump-start. If the alternator is not charging, the battery drains quickly and cannot sustain the engine.

Unusual smells or sounds
A burning rubber or electrical smell can indicate the alternator is overheating or the drive belt is slipping. A grinding noise may point to worn bearings inside the unit.

Confirming it is the alternator, not the battery

Before spending money on a replacement, it is worth confirming where the fault lies, because a dead battery and a failing alternator can produce similar symptoms.

The multimeter test
With the engine off, a healthy battery should read around 12 to 13 volts at the terminals. Start the engine and check again. A working alternator should push the reading up to between 13.5 and 14.5 volts. If the voltage stays at or drops below 12 volts with the engine running, the alternator is not charging properly.

The jump-start clue
If the car starts with a jump but dies again within a few minutes of running, the alternator is unlikely to be replenishing the battery. A brief run after a jump-start does not prove the alternator is healthy; have the charging system tested.

Load test
With the engine running, switch on headlights, the blower fan and rear demister. If the engine note changes noticeably or the lights dim significantly under load, the alternator is struggling to cope.

If you are unsure, most garages will test the charging system for a small fee or include it in a diagnostic check.

Typical repair costs in the UK

Alternator replacement costs vary considerably depending on the car, the type of replacement part and where you have the work done.

Parts
A new alternator for a mainstream family car typically costs between £150 and £350. Premium or performance models can push this higher, especially if the alternator is integrated with complex electronics or requires coding. Refurbished or reconditioned units are usually cheaper, often between £100 and £200, and can be a sensible option for an older car.

Labour
Labour charges generally fall between £150 and £250, depending on how accessible the alternator is and the garage's hourly rate. On some engines the alternator is straightforward to reach. On others it is buried behind other components, which adds time.

Total cost
According to WhoCanFixMyCar, the average alternator replacement price across UK garages is around £535, with typical jobs ranging from £250 for a simple swap on a smaller car to £800 or more for premium vehicles or dealership fitting.

Additional considerations
Ask whether the quote includes the drive belt, as this is often replaced at the same time. Some modern alternators also require software reset or coding after fitting, which may add to the cost.

Buying a replacement alternator

If you are sourcing the part yourself, either for a garage to fit or for a DIY job, there are several things to check before you buy.

Match the specification exactly
Use your car's registration, make, model, year and engine size to find a compatible unit. Alternators are rated by amperage output, and fitting one with the wrong rating can cause electrical problems or fail to charge properly.

New, refurbished or reconditioned
A brand-new alternator from a reputable manufacturer offers the longest warranty and peace of mind, typically 12 months or more. Refurbished units have been stripped, tested and had worn components replaced. They are cheaper and often perfectly adequate, particularly for a car you plan to sell within a couple of years. Avoid second-hand units sold without any testing or warranty, as you have no way of knowing how much life remains.

Check what is included
Some replacement alternators come with the pulley already fitted, which saves time. Others require you to transfer the pulley from your old unit. Confirm whether the part is supplied with a warranty and what it covers.

Where to buy
Motor factors, main dealers and online parts specialists all stock alternators. You can also browse car parts on Gumtree, but confirm part number, condition and seller details before buying. Compare prices across a few suppliers and check reviews before committing.

DIY fitting versus professional replacement

Fitting an alternator yourself is possible if you have mechanical experience and the right tools, but it is not a straightforward job for everyone.

When DIY makes sense
If the alternator is easily accessible, you have a socket set, and you are comfortable disconnecting the battery and working with electrical connections, you can save on labour. Always disconnect the battery negative terminal before starting, and label or photograph wiring connections before removing them.

When to use a garage
If the alternator is buried behind other components, requires special tools, or your car needs software coding after fitting, a professional job is the safer option. A garage will also test the charging system after installation and can spot related issues such as a worn drive belt or corroded connections.

Battery health
If the alternator has been failing for some time, the battery may also be damaged from repeated deep discharge. Ask the garage to test the battery as well, as you may need to replace both.

FAQ

Can I drive with a failing alternator?

Not for long. Once the battery is depleted, the car will stall and electrical systems will fail, including power steering and brakes on some modern vehicles. If you notice warning signs, get the car checked as soon as possible rather than risking a breakdown.

How long does an alternator typically last?

Most alternators last between 100,000 and 150,000 miles, or roughly 7 to 10 years, though this varies with driving conditions and electrical load. Cars with many power-hungry accessories or that do a lot of short-town trips may see earlier failure.

Is it worth replacing the alternator on an old car?

That depends on the car's overall condition and value. If the car is otherwise sound and you plan to keep it for a few more years, a replacement makes sense. For a car nearing the end of its life, a refurbished unit may offer the best balance of cost and reliability.

Will a new alternator fix a flat battery?

Only if the battery itself is still healthy. If the battery has been repeatedly drained by a failing alternator, it may no longer hold a charge and will need replacing as well. Have both tested together.

What is the difference between a reconditioned and a refurbished alternator?

The terms are often used interchangeably, but reconditioned usually means the unit has been fully stripped, inspected and had all worn components replaced to meet original specifications. Refurbished may involve less extensive work. Always check what the supplier's process includes and what warranty is offered.