Preventive vs corrective maintenance explained

Preventive maintenance is work scheduled before a failure to reduce its likelihood; corrective maintenance restores an asset after a fault has been found or it has failed.

Every maintenance strategy is a mix of two things: work you plan before a failure, and work you do because something already failed. The question is never "preventive or corrective?" — it is "what is the right mix for each asset?"

This article defines the main maintenance strategies, compares their costs and risks honestly, walks through a worked cost comparison, and shows how to decide which assets deserve a preventive plan. Whatever the mix, both kinds of work are easier to manage when every work order sits on the asset's own history — which is what maintenance management software is for.

Definitions

Corrective maintenance is work done to restore an asset after a fault has been found or it has failed. It includes emergency breakdown repairs, but also planned corrections of defects found during an inspection.

Preventive maintenance (PM) is work done on a schedule, before failure, to reduce the likelihood of failure. The schedule can be time-based (every 90 days), usage-based (every 500 operating hours) or a combination. See the glossary entry on preventive maintenance.

Two related strategies are worth knowing:

• Condition-based maintenance performs work when a measured condition — vibration, temperature, oil analysis, wear — crosses a threshold.
• Predictive maintenance uses trends in condition data to estimate when a failure is likely and schedule work just before it.

And one deliberate strategy that is often overlooked:

• Run-to-failure is a conscious decision to do no preventive work on an asset because failure is cheap, safe and quick to fix. A desk lamp is the classic example.

The honest comparison

Preventive maintenance is not free. Every PM task costs labour, parts and some downtime, and a poorly designed PM program can replace components that had plenty of life left. The goal is to do the tasks that genuinely prevent expensive failures, and no others.

A worked cost comparison

Illustrative example with assumed numbers. Replace them with your own.

An air compressor supports a small production line.

Run-to-failure scenario (per year):

• Expected breakdowns: 2
• Repair cost per breakdown (parts, call-out, labour): $1,800
• Lost production during each breakdown: 6 hours × $400 per hour = $2,400
• Annual cost: 2 × ($1,800 + $2,400) = 2 × $4,200 = $8,400

Preventive scenario (per year):

• Quarterly service: 4 × $350 = $1,400
• Planned downtime per service: 1 hour, scheduled outside production, so $0 lost output
• Expected breakdowns reduced to 0.5 per year: 0.5 × $4,200 = $2,100
• Annual cost: $1,400 + $2,100 = $3,500

To put your own number on lost production, the free downtime cost calculator turns lost output and idle labour into a cost per hour, per incident and per year.

In this example, PM saves $8,400 − $3,500 = $4,900 per year. The key sensitivities are the cost of lost production and how much the PM tasks actually reduce the failure rate. If the compressor had a standby unit, lost production would be close to zero, and run-to-failure would cost 2 × $1,800 = $3,600 — almost the same as the preventive plan. That is why the decision has to be made asset by asset.

Which assets deserve a PM plan?

Score each asset or asset class on four questions:

1. Consequence of failure. Does it stop production, create a safety hazard or breach a regulation?
2. Failure pattern. Does it wear out predictably (belts, filters, bearings)? PM works best on wear-related failures. Random electronic failures are rarely prevented by time-based servicing.
3. Cost of the PM task relative to the failure it prevents.
4. Redundancy. Is there a standby unit or an easy workaround?

High consequence plus predictable wear equals a strong PM candidate. Low consequence plus random failure equals run-to-failure. Everything in between is a judgement call — start with manufacturer recommendations and adjust based on your own failure history.

Building a basic PM schedule

For each PM candidate, define:

• The task — what exactly is done (inspect, lubricate, replace filter, calibrate).
• The trigger — calendar interval, usage interval, or both ("every 3 months or 500 hours, whichever comes first").
• The skill and time required.
• Parts and consumables.
• The acceptance criteria — what "pass" looks like, and what to do if it fails.

Our guide to building a preventive maintenance program covers scheduling, backlog management and metrics in depth.

Measuring the balance

A handful of metrics tell you whether your mix is right:

• Planned vs unplanned work ratio — the share of maintenance hours spent on scheduled work.
• PM compliance — the share of PM tasks completed on time.
• Mean time between failures (MTBF) for critical assets — should rise as PM takes effect.
• Maintenance cost per asset — to spot assets that cost more to keep than to replace.

Calculating the preventive to corrective maintenance ratio

Take one period, such as last month or last quarter, and add up the hours (or work orders) of each kind of work. Then:

• Preventive share = preventive hours ÷ (preventive + corrective hours) × 100
• PM : CM ratio = preventive hours ÷ corrective hours

Illustrative numbers: 120 hours of preventive work and 40 hours of corrective work give a preventive share of 120 ÷ 160 × 100 = 75% and a PM : CM ratio of 120 ÷ 40 = 3 : 1. Reaching an 80% preventive share at the same total of 160 hours would mean moving 8 hours from reactive to planned work.

A common rule of thumb is about 80% preventive to 20% corrective, but the right mix depends on your assets, for the reasons in the sections above: run-to-failure items pull the ratio down on purpose. Track the trend for your critical assets rather than chasing a single number. The free preventive vs corrective maintenance ratio calculator does the arithmetic, adds a cost split if you enter costs, and shows how far you are from your target.

None of these metrics is possible without a service history attached to each asset. That is the first thing to fix if you are purely reactive today: log every repair as a work order against the asset.

Common mistakes

• Over-maintaining. A PM task that does not reduce failures still costs time, parts and downtime, and it can replace components that had plenty of life left. Keep the tasks that prevent expensive failures and drop the rest.
• Time-based servicing for random failures. Fixed-interval PM works on wear: belts, filters, bearings. Random electronic failures are rarely prevented by time-based servicing, so a calendar task on those assets adds cost without reducing breakdowns.
• Repairs not logged against the asset. Without a service history on each asset you cannot measure MTBF, cost per asset or the preventive to corrective ratio, and you cannot tell which assets deserve a PM plan.
• Chasing a single ratio target. The 80/20 rule of thumb is a starting point, not a goal for every asset: run-to-failure items pull the ratio down on purpose. Watch the trend on your critical assets.
• Treating all corrective work as an emergency. Corrective maintenance also covers planned corrections of defects found during an inspection. Those can be scheduled with parts and people ready. Count them as planned work, but still as corrective in the PM : CM ratio.

Maintenance in Asetavo

Asetavo's maintenance module links every work order — with status, cost and parts — to the asset, so the Maintenance tab shows its complete history. Preventive schedules produce a due list of upcoming work today; automatic work-order generation from schedules is on the roadmap. Calibration is logged through work-order completion, and reports show maintenance cost by asset so repair-or-replace decisions use real data. Maintenance is part of the Business plan — see pricing.

Related reading: CMMS in the glossary, what is asset management?, and the free maintenance ratio calculator.