Should I change all 4 glow plugs?

When it comes to glow plug maintenance in a diesel engine, a common question that arises is whether you need to change all 4 glow plugs at once, or if you can just replace one or two that seem to be malfunctioning. In this article, we’ll explore the ins and outs of glow plugs, look at the pros and cons of replacing just one or two vs. all four, and provide a definitive answer on the best practice for glow plug replacement.

What are glow plugs and what do they do?

Glow plugs are small heating elements that help get a diesel engine started in cold weather. Diesel fuel doesn’t ignite through compression like gasoline does, so it needs the extra heat from glow plugs to raise the temperature inside the cylinders enough for combustion to occur. When you turn the key in your diesel vehicle, a current runs through the glow plugs for a few seconds to heat them up, before the starter motor cranks the engine over. The hot glow plugs initiate combustion, allowing the engine to start.

Once the engine warms up and is running smoothly, the glow plugs turn off. But they turn on again when needed, such as during a cold restart of the engine. So in short, glow plugs help heat up the diesel combustion chambers to aid starting.

What are the signs of failing glow plugs?

Some common signs that your glow plugs may be failing and need replacement include:

  • Difficulty starting the vehicle, particularly when cold outside
  • Rough idle or misfiring right after starting
  • A “check engine” or “glow plug” warning light coming on
  • Poor fuel economy and acceleration
  • Excessive smoke from the tailpipe, especially when cold
  • Failed glow plugs found during inspection

Glow plugs deteriorate over time due to factors like combustion chamber deposits, electrical damage, and corrosion. Typically they need replacement between 60,000-100,000 miles, or earlier if problems arise.

Should I replace just one failed glow plug?

When one glow plug goes bad, it’s tempting to just replace that single plug instead of doing all four. However, there are a few reasons why it’s usually better to replace all the glow plugs as a complete set:

  • The other glow plugs are likely close in age and condition to the failed one. If one went bad, the others may not be far behind.
  • You don’t know for sure if only one is bad or if the others are also on the verge of failing.
  • Mixing an old glow plug with new ones can cause uneven heating and performance.
  • It’s convenient to replace them all at once rather than spreading out single plug replacements.
  • New glow plugs will maximize starting performance.

Replacing only the visibly bad glow plug while leaving the marginal ones in place could just mean you have to service them again much sooner. It’s better to be thorough and get all new ones in there, so you can be confident they are all in top working condition.

Drawbacks of replacing one glow plug

Some potential downsides of only replacing a single failed glow plug include:

  • Other worn glow plugs could fail soon after
  • Uneven heating and performance with mixed old and new plugs
  • Difficulty diagnosing if there is another bad one still in the engine
  • Shorter service life than replacing the full set
  • More labor for multiple single plug replacements over time

Benefits of replacing all 4 glow plugs

Some benefits of replacing all four glow plugs simultaneously include:

  • Ensures all are in good functioning order
  • Balances heating performance across cylinders
  • Maximizes cold starting ability
  • Longer service life than just replacing one
  • Fewer future maintenance issues
  • Less labor compared to multiple individual replacements

Is it absolutely necessary to replace all the glow plugs?

While you can get away with replacing just a failed glow plug temporarily, for optimal performance and reliability, replacing all the glow plugs as a complete set is highly recommended. Here are some reasons why:

  • The glow plugs are designed to work in sync together, so uneven or reduced heating from old plugs will affect performance.
  • Leaving in weak glow plugs can allow unburned fuel to build up and damage the exhaust system over time.
  • The labor cost for one plug job versus all four is usually only marginally more.
  • It avoids the hassle of having to inspect and potentially replace individual plugs again down the road.
  • All engines will benefit from having the maximum cold starting capability.

While not absolutely mandatory, to get the reliability, service life, and performance glow plugs are designed for, replacing the full set together is highly recommended by manufacturers and mechanics alike.

Replace One Glow Plug Replace All 4 Glow Plugs
Saves slightly on upfront cost Ensures all plugs are in good condition
Other old plugs could still fail Balances heating across cylinders
Uneven performance Optimizes cold starting
Short term solution Longer lasting glow plug system
Potential recheck and reservice needed sooner Fewer future maintenance issues

How expensive is it to replace all 4 glow plugs?

The parts cost for a full set of OEM direct replacement glow plugs ranges from $40-120 depending on the vehicle make and model. Aftermarket glow plugs can lower the parts cost a bit as well. Labor costs typically add another $150-250 to the overall bill, since some disassembly is required around the cylinders to access the plugs.

So in total you’re looking at around $200-350 to have a professional replace all four glow plugs with new ones. The upside is this service generally won’t need repeating for around 60,000 miles of driving once done.

Replacing just one glow plug averages $50-100 or more for parts and labor. However, you are likely to encounter the remaining old plugs failing and requiring service again much sooner. In the long run, you’ll spend less by doing the complete set all at once.

What are the steps to replacing glow plugs?

Replacing all four glow plugs involves the following general process:

  1. Evaluate engine temperature conditions and prepare the vehicle – The engine should be completely cooled off to avoid burns. Disconnect the battery to deactivate the fuel system.
  2. Gain access to glow plugs – Coils, injectors, tubes, and other engine components may need removing to access the tops of the cylinders.
  3. Extract old glow plugs – Use a socket wrench to unscrew and remove the old glow plugs from the cylinder head one by one.
  4. Install new glow plugs – Screw in the new plugs by hand until seated, then tighten with a torque wrench to specification.
  5. Replace removed components – Reinstall any coils, injectors, electrical connectors, etc. that were detached for access.
  6. Reconnect battery – Restore battery power so vehicle computer can reset and test glow plug system.
  7. Start engine – Turn ignition and allow new glow plugs to run through warmup cycle to confirm operation.

It’s important to have a service manual on hand and use proper tools to avoid engine damage or electrical shorts when doing glow plug service. Take care not to overtighten plugs during installation as well.

Can I change glow plugs myself?

Replacing glow plugs is an advanced task, but can potentially be DIY for someone with mechanical experience. Things to keep in mind if attempting it yourself:

  • You’ll need automotive tools – ratchets, torque wrench, socket set, plug socket, etc.
  • Follow the factory service procedure exactly.
  • Be very cautious handling engine wiring and connectors.
  • Work slowly and carefully to avoid damaging threads or connectors.
  • Double check tightness specifications for reassembly.
  • Dispose of old glow plugs properly – they may contain asbestos.
  • Be prepared for a lengthy, difficult process – factor in 4+ hours.

If you don’t have extensive experience doing your own engine service, glow plug replacement is probably best left to a professional mechanic. They’ll have the tools, expertise, and efficiency to swap the glow plugs quickly and safely.

Tips for DIY glow plug replacement

If you decide to take on replacing glow plugs yourself, these tips can help it go smoothly:

  • Allow plenty of time – don’t rush through it.
  • Follow the factory service manual exactly.
  • Label and bag components carefully while disassembling.
  • Check glow plug wire boots for cracks or damage.
  • Have the engine stone cold before starting.
  • Wear protective gloves and eye wear.
  • Use a plug socket rather than pliers to avoid damage.
  • Apply anti-seize compound to plug threads before installing.
  • Torque new plugs to spec – don’t overtighten!


Replacing all your glow plugs as a set rather than individually as they fail is the preferred practice. While you can get by swapping only a bad plug initially, you’ll get longer service life, prevent future issues, and maximize performance by replacing them all together. For most vehicles, the labor difference is reasonable for the long term benefit. And Do-It-Yourself mechanics can perform the replacement if armed with the right tools, patience, and repair manual. Just be sure to handle components carefully and torque all glow plugs to the factory specification.

Leave a Comment