Comparison image showing OEM parts versus aftermarket parts in a workshop setting, with brake pads, oil filters, spark plugs and boxed components, used for a blog about choosing the right vehicle parts by Mid Ulster Rotating Electrics.

OEM vs Aftermarket Parts: How to Save Money and Stay Reliable

Introduction

If you’ve ever turned the key on a cold, damp morning and heard nothing but that depressing “click… click…”, you’ll know the panic that comes when a starter motor or alternator decides to give up. And once the breakdown is sorted, you’re left with the next headache:

“Do I buy the genuine part… or save money with an aftermarket one?”

It’s one of the most common questions we get at Mid Ulster Rotating Electrics and the truth is, the right answer depends entirely on which parts you’re comparing.

This guide breaks down the OEM vs Aftermarket parts debate in clear, simple language so you can make a confident, cost-effective decision the next time a vital component fails.

 

What Are OEM Parts? (The Gold Standard)

OEM stands for Original Equipment Manufacturer.
These are the exact same parts fitted to your car when it rolled out of the factory.

Pros of OEM Parts

  • Perfect fitment: No adjusting, no guessing, no hassle.
  • Guaranteed performance: Built to the manufacturer’s exact spec.
  • Warranty friendly: Ideal for newer vehicles and MOT compliance.
  • Known reliability: Especially important for complex electrics.

Cons of OEM Parts

  • Expensive: OEM parts often cost significantly more.
  • Lead times: Main dealers sometimes need to order parts in.
  • Brand tax: Many OEM parts are made by Bosch/Valeo but sold by dealers at a heavy markup.

Real Example

Take an alternator: the OEM version is designed to handle the exact electrical load of your vehicle, including modern demands like heated seats, LED lighting, stop/start, and infotainment systems. A poorly built copy can struggle, overheat, or fail prematurely.

 

What Are Aftermarket Parts? (The Mixed Bag)

Aftermarket parts are made by third-party manufacturers.
They range from top-tier engineering to cheap knock-offs and everything in between.

Pros of Aftermarket Parts

  • More affordable: Ideal for older vehicles.
  • Widely available: Usually in stock, unlike some dealer items.
  • Upgrades: Some aftermarket brands fix known OEM weaknesses.

Cons of Aftermarket Parts

  • Quality varies massively.
  • Compatibility issues: Poorly made components can cause warning lights or electrical faults.
  • MOT concerns: Cheap vehicle lighting, for example, may fail a beam pattern test.

The “Improved Design” Advantage

High-quality aftermarket brands sometimes upgrade internal components.
A good example is a starter motor where the aftermarket manufacturer uses:

  • heavier-duty brushes
  • better bearings
  • improved solenoids

This can actually outperform the original design.

 

The OE Supplier Secret (The Cheapest Way to Get the Dealer Part)

Here’s something many drivers don’t know:

Most car brands don’t make their own electrical components.

They buy them from major suppliers like:

  • Bosch
  • Valeo
  • Denso
  • Mitsubishi Electric

So when you buy a “genuine dealer” starter or alternator, you’re mostly paying extra for the box, not the part.

The Smart Option

Buying the same Bosch/Valeo/Denso unit from Mid Ulster Rotating Electrics gives you:

  • The identical part
  • The same fit, same performance
  • A much lower cost
  • No long dealer wait times

This is why OE-supplier parts are considered the true best of both worlds.

 

Repair vs Replace - An Important Consideration

Sometimes the smartest choice isn’t new or aftermarket…

It’s repairing or reconditioning your existing OEM unit.

At Mid Ulster Rotating Electrics, we specialise in:

Why Repairs Are Often Better

  • OEM housings and cores are usually superior to cheap replacements.
  • Keeping the original component preserves vehicle reliability.
  • Reconditioning is environmentally friendly.
  • Repairs can be cheaper than buying new without sacrificing quality.

For example, a failing alternator may only need:

  • a new voltage regulator
  • fresh bearings
  • slip ring refurbishment

No point replacing the entire unit with a cheap import when the original is repairable and better built.

 

The Risks of Cheap “White Box” Internet Parts

You’ve seen them online, suspiciously cheap parts with no brand name.

Why They’re a Problem

  • Low-grade copper windings
  • Weak solder joints
  • Poorly machined housings
  • Overheating risks
  • Voltage instability
  • Very short lifespan

For Critical Parts Like Starters & Alternators:

Cheap units can cause:

  • Battery drain
  • ECU warnings
  • Voltage spikes
  • Complete breakdown
  • Fire risks in rare cases

Lighting Warning

Cheap LED bulbs can:

  • Fail MOT beam tests
  • Dazzle oncoming traffic
  • Interfere with CANBUS systems
  • Burn out quickly

The short-term savings rarely justify the long-term problems.

 

Warranty, MOTs & Insurance - What You Need to Know

Many drivers think using aftermarket parts voids warranties.

Good news: it doesn’t.

Under the Block Exemption Regulation (BER), you can use OE Quality parts (like the ones sold by us) without voiding manufacturer warranties.

However…

Using incorrect or extremely cheap parts can cause issues, such as:

  • electrical faults
  • failed diagnostics
  • premature component wear

MOT testers will also fail poor-quality lighting or incorrectly performing electrical components.

 

Side-by-Side Comparison Table

Feature

OEM (Dealer)

OE Supplier (What We Stock)

Cheap Internet Parts

Price

High

Medium

Very low

Quality

Excellent

Excellent (same manufacturer)

Unpredictable

Reliability

Guaranteed

Guaranteed

Often poor

Fitment

Perfect

Perfect

Hit-and-miss

Warranty Friendly

Yes

Yes (OE Quality)

Usually no

Availability

Sometimes slow

Usually fast

Fast

Best For

Newer cars, warranty

Everyday drivers wanting value

Only non-critical items

 

When Should You Choose OEM or Aftermarket?

Choose OEM / OE-Supplier / High-Quality Aftermarket When:

  • It’s a starter motor or alternator
  • It affects charging, starting, safety, or electronics
  • You need reliability for daily driving
  • You want worry-free performance

Choose Repair or Reconditioning When:

  • The OEM unit is superior to aftermarket copies
  • Your part is rare, expensive, or discontinued
  • You want a sustainable, cost-effective solution

Choose Budget Parts Only When:

  • It’s a purely cosmetic or low-impact part
  • e.g., interior trim, floor mats, clips
  • Never for critical electrics
  • Never for MOT-sensitive components like lights

 

Conclusion

Choosing between OEM vs Aftermarket parts doesn’t have to be confusing. For critical components like starters & alternators, quality matters far more than the badge on the box.

At Mid Ulster Rotating Electrics, we stock:

  • genuine OE-supplier parts
  • high-quality aftermarket options
  • remanufactured units
  • full repair services

…so you get the right solution for your vehicle and your budget.

Whether you need a new unit, a repair, or information on what’s best, our team is here to help, Visit our Contact Us page if you ever need any advice.

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