Preventive maintenance schedules: by mileage, engine hours or date

What the rules require, and what they leave to you

The federal rule is short. Under 49 CFR 396.3(a), every motor carrier must "systematically inspect, repair, and maintain" all motor vehicles under its control, and parts and accessories must be "in safe and proper operating condition at all times." Section 396.3(b) adds that the records for each vehicle must include "a means to indicate the nature and due date of the various inspection and maintenance operations to be performed."

That is the whole of it. The regulation does not set oil change intervals, name service levels, or say how many miles may pass between inspections. A written schedule with due dates is how most fleets show that their maintenance is systematic, but the intervals themselves come from you. See maintenance records for what the file must hold and how long to keep it.

Where the intervals should come from

• The manufacturer's maintenance manual first. Engine, transmission and axle makers publish service intervals, often with different tables for normal and severe duty. Following them also protects warranty coverage; see fleet warranty claims.
• Your duty cycle second. Short-haul stop-and-go work, long idle periods, dusty sites and heavy loads usually fall under the severe-duty column. Many fleets place a vehicle in one column and record why.
• Your own history third. Repeat failures between services suggest the interval or the checklist is wrong for that vehicle. Oil analysis is one common way fleets test whether an interval can be extended or should be shortened.

Service levels: A, B and C

Many fleets group work into service levels, often called A, B and C (some use more). The letters are a convention, not a standard, and what goes into each level varies from fleet to fleet. A common pattern:

Some fleets time a C service to coincide with the annual DOT inspection, so the vehicle is off the road once instead of twice. That is a scheduling choice; the annual inspection is a separate requirement with its own report.

Miles, engine hours or months: whichever comes first

A single counter misleads. A truck that idles for hours at a job site wears its engine without adding miles; a trailer or a lightly used van can sit for months and still need its fluids and rubber parts changed. So most schedules set two or three limits for each task and trigger on the first one reached:

• Distance (odometer miles) for on-road wear: tires, brakes, driveline.
• Engine hours for engines that idle or power equipment, and for off-road machines with no meaningful odometer.
• Time (days or months) for anything that ages whether or not it is used: fluids, filters, batteries, hoses, and inspections with a calendar deadline.

Count each interval from the reading at the last completed service, not from a fixed calendar. If an A service is every 15,000 miles and it was done at 92,400, the next is due at 107,400, even if the one before was late. A schedule that counts from fixed points quietly shortens or stretches intervals every time a service moves.

Distance and hour limits are only as good as the readings behind them. A schedule that has not had an odometer reading for a month cannot tell you whether a vehicle is due. Treat a stale reading as its own problem to fix, not as "not due."

Due-soon windows

A schedule that only says "due" leaves no time to plan. A common approach is a due-soon window for each counter: for example, two weeks before the date, a few hundred miles before the distance, or a day or two of running before the hour limit. The window gives the shop time to order parts, book a bay and arrange a spare vehicle. The right size depends on how fast a vehicle runs up its counters; a vehicle covering 3,000 miles a week needs a wider mileage window than one covering 300.

When a PM is missed

1. Record that it is overdue, and by how much. Do not move the due point to make the list look clean.
2. Decide whether the vehicle can keep running. A late oil change is not the same as a late brake inspection. Anything that affects safe operation should be checked before the vehicle's next trip.
3. Do the work and record the actual reading. The next interval counts from that reading.
4. Find the cause. Missed services usually come from stale readings, vehicles that are hard to pull from service, or nobody owning the list. Fix the cause, not just the vehicle.

A schedule checklist

• Every vehicle has a schedule; none is missing because it is new, borrowed or rarely used.
• Each task has its source (manual, duty cycle, your own decision) written next to it.
• Each task has distance, hour and time limits where they apply, and triggers on the first.
• Readings are recorded often enough that "due" can be trusted.
• Overdue items are visible to the person who can take the vehicle off the road.
• Completed work records the date, the reading and what was done, ready for an inspection or an audit.

Cargovate schedules service by mileage, engine hours or date, whichever comes first, shows what is due soon and overdue for each vehicle, and raises an alert when a service goes overdue. For the terms used here, see the glossary.