Because both involve efforts to reduce unplanned downtime, it’s tempting to think of maintenance management and asset reliability as interchangeable terms. But you need to know not only the differences but also the connections so your maintenance team and reliability professionals can deliver an effective, efficient asset management program with a strong return on investment throughout all stages of the life cycle. 

What is the definition of maintenance? 

For all the possible complexity, you can still boil maintenance down to a simple imperative:  

Retain and restore function.  

An organization needs its physical assets and equipment to perform specific functions, and maintenance is the collection of people and processes that keep everything up and running. There are two main categories for maintenance strategies.

In some cases, the maintenance department works proactively, using inspections and tasks to retain an asset’s function. For example, monitoring and changing an engine’s oil. In others, they are in a more reactive mode, jumping in only after a failure. Here, maintenance techs do everything from swapping out old light bulbs to replacing a blown gasket to restore assets to their desired level of function. 

You can also summarize maintenance with a three-letter initialism: MRO, which stands for maintenance, repair, and overhaul. That covers all the: 

  • Inspecting 
  • Testing 
  • Measuring 
  • Servicing 
  • Adjusting 
  • Replacing 
  • Rebuilding 

An important part of the definition is that you only perform maintenance during the utilization period of an asset’s life cycle. You do maintenance when an asset is up and running, and the goal is to keep it like that cost-effectively for as long as possible.  

Maintenance can be a consideration at other stages, though. For example, some of the layout of a car’s engine is dictated by the need to periodically check the oil and add various fluids. And repair-or-replace decisions always involve a close look at how many resources you’re putting into maintaining an asset.

When deciding which assets to purchase, organizations look closely at maintainability, which is the ability to maintain or restore an asset. If it stops working, how hard is it to find and fix the problem?  

There is also a lot of value in using building information modeling for facility management (BIM for FM), where data from the design and construction phases of a facility are repurposed for operations and maintenance.

So, for example, you take all the data related to installing the plumbing and make it both relevant and available to the maintenance teams. But, if you’re looking strictly at the definition of maintenance, it’s only for an asset’s useful life.  

What is the definition of reliability? 

Some terms can get confusing because they have a general and an area-specific definition. For example, the word “theory” has a different meaning in common usage than it does in science, where you can talk about the “theory of gravity.” Reliability is the same. When you talk about a reliable friend, it’s different than when you describe a reliable asset or piece of equipment.   

In maintenance, reliability is the ability of an asset or piece of equipment to perform a function under specific operating conditions over a time interval. For maintenance, you’re focused on maintaining an asset’s ability to perform a function.

With reliability, you expand your scope to include operating conditions and time. So, a good working definition of reliability is “The probability that an asset (or some part of it) will perform the required function for a specific time under defined operating conditions.” 

Basically, reliability is the chance that an asset will work the way you want it to for as long as you need it, under predetermined operating conditions. You can think of reliability as a measure of dependability. Is it there when you need it? Does it work the way it should? How much time is there between failures? Can you trust there will be close to zero unplanned downtime?  

That last question might be the most important one. In fact, one way to think of reliability is as the absence of unplanned downtime. 

Reliability is not availability 

They both sound like the answer to the question “Is it there when you need it?” But as a maintenance metric, availability is not the same as reliability. Instead, it is a way of measuring the percentage of time you can use an asset. For example, when you take a road trip, your car is not available the entire time.

There’s the time you’re behind the wheel, but there’s also the times you stop for gas, check the oil and tire pressure, a top up the window washer fluid. If you get a flat or the engine runs into trouble, you lose that time, too. You might be traveling for eight hours, but because all the preventive maintenance, repairs, and refueling took 2 hours, your availability was only 6 hours. 

So, reliability and availability are different, but they’re related. If you want to increase availability, you need to look at boosting reliability. In the road trip example, you’re always going to need to pit stops to do some minor preventive maintenance and refuel, but by decreasing downtime, you get more availability.  

Is it maintenance vs reliability or maintenance and reliability? 

It’s neither. It makes more sense to say maintenance falls under reliability. 

Asset management is everything an organization does to get the most from its assets, over the entire life cycle. It encompasses all the work that goes into choosing which assets to purchase, how to operate and maintain them, and when and how to retire and replace them. Asset management is cradle to grave.  

Reliability is one part of asset management, and it’s everything an organization does to boost reliability and availability within operating conditions. Maintenance is one part of your reliability efforts. It is what maintenance techs do to keep assets up and running. 

What is an example of maintenance techs and reliability professionals working together? 

At a fulfillment warehouse, the maintenance team reviews its work order records and decides one of the conveyor belts is failing at a higher rate than all the others. To cut unplanned downtime, the team starts by increasing the number and frequency of inspections and tasks on the existing preventive maintenance schedule.

If they used to check the conveyor belt once every two weeks, they now start inspecting it once every 10 days. Later, they might add more steps to the list of inspections and tasks, dedicating more time to adjusting the tension and ensuring all the parts are properly lubricated. Eventually, they can start to look at the lubrication. Switching to a different make or line might be the solution.

The maintenance team can also make changes to how many associated spare parts and materials they carry in stock. By having more on hand, they can likely complete repairs more quickly after failures. 

Reliability engineers can look for solutions using a wider scope, including the conveyer’s: 

  • Specifications 
  • Operation 
  • Maintenance 

So, instead of being limited to fine-tuning the type and timing of maintenance, the reliability team has more options. If they decide that the root cause of the problem is the weight or number of boxes on the belt, they can make changes to the operating conditions.

If the existing standard operating procedure is to have X number of boxes on the belt at any given time, they can lower it to Y. The belt might move fewer boxes over the course of any given day, but the reduction in both scheduled maintenance and unplanned downtime might deliver better overall numbers.   


Although both are parts of the larger project of asset management, reliability and maintenance are different. They both share a focus on keeping assets and equipment up and running, but the scope and approaches are different.

A good working definition of maintenance is the combination of people and processes that an organization uses to reduce unplanned downtime. They can be reactive, such as when a maintenance department relies on run-to-failure. Or they can be proactive. For example, a preventive maintenance program. It’s important to remember that maintenance activities focus on the utilization period for an asset.

Reliability, on the other hand, is an asset’s ability to perform a function under predetermined operating conditions over a time interval. Maintenance is a part of reliability. So, if a maintenance team wants to reduce downtime for an asset, they can fine-tune the schedule of inspections or look at stocking different or more associated parts and materials.

But reliability professionals can look at not only the maintenance but also the operating conditions. For example, they can change the design of an asset or run it at a different rate. Maintenance and reliability are different, but you need both for an effective, efficient asset management program. 

About the author

Jonathan Davis

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