Measuring and fixing PM compliance

PM compliance is the share of scheduled preventive maintenance completed within its interval. It is the closest thing fleet maintenance has to a leading indicator: when it drops, unplanned downtime and repair cost rise a few months later, reliably enough that you can forecast on it.

It is also the metric most commonly reported as a number nobody believes.

Defining it honestly

PM compliance = PM services completed on time ÷ PM services that came due, over a period.

Three decisions determine whether the result means anything:

The window. A common standard is completion within ±10% of the interval — a 5,000-mile PM counts as on time between 4,500 and 5,500 miles. Some fleets use a flat window (within 250 miles or 7 days). Tighter windows produce lower numbers and better maintenance.

What counts as due. A unit that was out of service for a month did not skip its PM. Decide whether to exclude out-of-service periods. Excluding is defensible; not documenting the exclusion is not.

Whether you count deferrals. If a PM is deferred with approval, is it on time? The honest answer is no — deferral is the thing you are trying to see. Track approved deferrals separately rather than removing them from the denominator.

Setting the intervals themselves

Compliance against the wrong interval is worthless.

Start from the manufacturer’s schedule for the specific duty cycle — most OEMs publish severe-duty schedules alongside normal, and most municipal work is severe duty by their definition. Stop-start operation, extended idling, heavy loads, and corrosive exposure all qualify.

Then adjust from your own evidence. Oil analysis is the cheapest way to find out whether your intervals are right; a few samples per class will tell you more than any published table. Where a unit consistently shows clean results at interval, extending is a legitimate cost saving. Where it does not, you have found a problem before it became a failure.

The hour-meter point matters more than it looks. A vehicle that idles heavily accumulates engine wear that the odometer does not see. Scoring its PM on miles alone means servicing it far past the equivalent wear point.

Raising it when the shop is already behind

The unhelpful advice is “schedule better.” If compliance is at 60%, the shop is not behind because nobody thought of scheduling.

Find out whether it is capacity or scheduling. Count PM hours due per month against available technician hours. If demand exceeds capacity, no scheduling change fixes it — that is a staffing, outsourcing, or interval problem, and you should be making that case rather than pushing the shop harder.

Protect PM capacity explicitly. The failure mode is PM slots being consumed by breakdowns. Reserving fixed capacity — two bays, or Tuesday and Wednesday mornings — feels impossible when you are behind, and it is the only thing that breaks the cycle. Unplanned work expands to fill whatever you give it.

Fix the availability problem, not just the shop. A large share of missed PMs are units that never showed up. That is an operations problem: the department needs the truck, the driver forgot, nobody told them. Booked appointments with the department, and a report to supervisors of units overdue, moves this more than anything you can do in the shop.

Split the PM. If a full B-service takes a unit out for a day the department cannot spare, a shorter A-service done reliably beats a thorough one done occasionally.

What to expect

Published survey figures suggest fully scheduled maintenance environments are rare, with most fleets running a substantial share of unscheduled and emergency work. Those figures come from vendor surveys and should be treated as directional — the sampling is not disclosed.

Your own trend is the number that matters. Measure monthly, plot it, and watch what happens to unplanned downtime two to three months after compliance moves. Once you can show that relationship in your own data, PM capacity becomes a much easier thing to fund.

The compliance angle

For fleets under federal motor carrier rules, a systematic inspection, repair and maintenance programme is not optional — it is required by 49 CFR 396.3, with records retained per vehicle. In Canada, National Safety Code Standard 11 requires a formal preventive maintenance programme.

In both cases an auditor is checking whether the programme exists, whether it is being followed, and whether the records prove it. PM compliance is the number that answers all three, which is a useful thing to point out when you are asking for the capacity to improve it.