Most published guidance on CNG fuel system inspection quotes the same figure — every 36 months or 36,000 miles — and for a refuse truck that figure is simply wrong. The interval in the container label wording required by FMVSS No. 304 splits on vehicle weight, and a garbage truck sits firmly on the heavy side of it, where the requirement is at least every 12 months. Both numbers appear in the same sentence, which is exactly how the confusion spread, and fleets that adopted the 36-month figure in good faith are running two thirds of a cycle behind on a pressure vessel. The rest of CNG maintenance is well understood and manageable; this one detail is worth checking today. See a cylinder inspection logged against a truck.
Your CNG Tanks Are on a Shorter Clock Than You Think
For fleet managers, municipal directors and shop supervisors running CNG refuse: the inspection interval that actually applies to a heavy truck, the four levels of inspection and who is allowed to perform each, and what changes in the workshop and the building — not just on the vehicle.
The Interval That Splits on Vehicle Weight
Here is the label wording itself, which is the clearest statement of the requirement and the reason the confusion exists — both intervals appear in the same sentence.
"This container should be visually inspected for damage and deterioration after a motor vehicle accident or fire, and either (a) at least every 12 months when installed on a vehicle with a GVWR greater than 4,536 kg, or (b) at least every 36 months or 36,000 miles, whichever comes first, when installed on a vehicle with a GVWR less than or equal to 4,536 kg."
— Container label wording required by FMVSS No. 304, as revised
Read the sentence carefully and three separate obligations fall out of it. First, an accident or fire triggers an inspection regardless of where you are in the cycle — that clause sits ahead of both intervals and applies to every vehicle. Second, the heavy-duty interval is annual. Third, the mileage limit only attaches to the lighter category, which means a heavy truck cannot inspect late on the grounds that it has covered few miles. A refuse truck that does 12,000 miles a year is still on the 12-month clock.
Take your own compliance advice on exactly how this applies to your vehicles and jurisdiction. The practical point stands regardless: if your CNG inspection schedule was built from the 36-month figure, check which category your trucks actually fall into before the next one comes due.
Four Levels of Inspection, and Who Can Do Each
"Inspection" covers four different activities with different depth and different qualification requirements. Conflating them is how fleets end up believing a walkaround satisfied the annual obligation.
Before the vehicle enters service or after any container re-installation.
Confirms the system was installed and commissioned correctly.
Routine, frequent, informal.
Can be done by the operator or a trained technician — a look for obvious damage, leaks or loose mounting.
Broader visual check of the system.
Still within the reach of a trained technician, but covering more of the fuel system than a walkaround.
The one the interval is really about.
Must be done by someone certified by an outside body or formally qualified by the fleet, and covers fifteen major system categories.
The qualification line is the one to plan around. A detailed inspection has to be carried out by someone certified by an outside entity or formally qualified by the fleet operator — a person who understands CNG components, container types, allowable damage limits and the inspection procedure itself. That is a training decision with lead time, and it is a records obligation afterwards, because being able to show who was qualified and when matters as much as the inspection result.
What the Inspection Is Looking For
A detailed inspection covers fifteen major system categories, and the containers are only one of them. The parts that most often produce findings are the ones exposed to the road.
Containers and their shields are checked for abrasion, impact damage, cuts, chemical attack and heat exposure — and for composite cylinders the allowable damage limits are specific and published, which is precisely why the work needs a trained eye rather than a general opinion. Mounting brackets and straps are checked for corrosion and looseness, since a container that can move is a container that will abrade. Beyond the tanks sit the valves, regulators, filters, high- and low-pressure fuel lines, fittings, the pressure relief device and its vent line, and the fill receptacle, all of which are inspected for damage, leaks and secure support.
On a refuse chassis the exposure is worse than average. The vehicle spends its day at low speed among containers, kerbs and debris, reverses constantly, and operates in an environment with corrosive leachate underfoot — so mounting corrosion and shield abrasion deserve more attention here than the generic guidance implies.
When is each of your CNG trucks next due?
If answering that means walking the yard and reading labels, the schedule is not really a schedule. In thirty minutes we will set up one truck with its containers tracked individually, the annual clock running, and an accident automatically reopening the inspection.
How FleetRabbit Handles This
Four things, and the second is the one that breaks most systems — because containers and vehicles are not the same asset.
Container inspection scheduled per vehicle on the heavy-duty interval, so it comes due on its own rather than when somebody notices a label.
Containers outlive installs and move between vehicles. Tracking them individually — with the service timeframe from the label — is the only way the inventory stays true.
A recorded collision or fire triggers the out-of-cycle inspection automatically, because that requirement does not wait for the annual date.
Detailed inspections need a certified or fleet-qualified person. Holding who is qualified, and until when, turns that from an assumption into a record.
What Else Changes Against a Diesel
Six differences, and the fifth one is a building project rather than a maintenance task.
- Fuel filters, not water separatorsCNG systems use high- and low-pressure filters that trap oil carryover and particulates from the compression process. Different part, different interval, same consequence if skipped.
- Oil change intervals often shortenNatural gas engines can be harder on oil than a comparable diesel, and they need an oil formulated for gas engines rather than whatever is on the diesel shelf. Using the wrong oil is a quiet, expensive mistake.
- Spark ignition returnsSpark plugs and coils are back on the PM sheet after a generation of diesel technicians never touching them, and on a high-hour refuse duty cycle they are a real recurring item.
- No DPF, no DEF, no SCRThe aftertreatment burden that makes refuse hard on diesels largely goes away, which is a meaningful part of why fleets moved to CNG in the first place.
- Methane detection in the shopThis is a building change, not a vehicle change. Indoor maintenance and parking facilities handling CNG need gas detection and appropriate ventilation, because methane rises and collects at the roof.
- Defueling before major workSome work requires the system depressurised and the vehicle defueled, which is a procedure with its own equipment and its own time cost that a diesel job never needed.
The facility point deserves emphasis because it is the one that cannot be solved later with a purchase order. Methane is lighter than air, so it collects at the highest point of a building rather than pooling at floor level like diesel vapour — which means detection, ventilation and heater placement all have to be designed for it. A workshop that was safe for diesel is not automatically safe for CNG, and this is a question for a qualified engineer rather than a maintenance manager.
Symptoms and Where to Start
Six things crews actually report, what usually sits behind each, and the first move. The fifth one is the most commonly misdiagnosed, because the fault is not on the vehicle at all. Swipe on mobile.
| What gets reported | Usual cause | Where to start |
|---|---|---|
| Loss of power, or running on fewer cylinders | Failing coil or spark plug, or a fuel filter restricting flow | Spark ignition is back on the PM sheet — check plug condition and interval before chasing fuel |
| Hard starting when cold | Regulator or pressure issue, or a weak ignition system under the extra demand | Check delivery pressure at the regulator before replacing ignition parts |
| Range dropping without a leak | Fill station pressure or temperature compensation, not the truck | Compare a fill at a different station before diagnosing the vehicle |
| Audible hiss or gas smell | A leak at a fitting, line or valve | Stop, ventilate, do not use ignition sources, and treat as a detailed-inspection trigger |
| Frequent regulator or filter replacement | Oil carryover from the compression process at the fill station | The truck is the symptom; the station is where the problem lives |
| Corrosion around mounts and shields | Refuse duty — leachate, debris and constant low-speed exposure | Raise it at the inspection rather than noting it; mounting integrity is a safety item |
The fourth row is not a diagnostic exercise. A suspected gas leak is a stop-and-secure situation, and it belongs in your written procedure with the same seriousness as a brake failure — not in a queue behind other work. Every other row can wait for the shop; that one cannot.
Questions CNG Fleets Ask
How often do CNG tanks need to be inspected?
For a vehicle over 4,536 kg GVWR — which includes refuse trucks — at least every 12 months, per the container label wording required by FMVSS No. 304. The widely quoted 36-month or 36,000-mile figure applies to lighter vehicles. Both intervals appear in the same sentence, which is how the confusion started. Put the annual clock on free.
What if the truck has been in an accident?
Inspect it, regardless of where you are in the annual cycle. The requirement to inspect after a motor vehicle accident or fire sits ahead of both intervals in the label wording and applies independently of them. Treat a fire or a significant impact as an automatic trigger rather than a judgement call. Set the trigger up with us.
Can our own technicians do the inspection?
For cursory and general checks, a trained technician or the operator can. A detailed inspection must be done by someone certified by an outside entity or formally qualified by the fleet, with real knowledge of container types and allowable damage limits. Plan that training ahead of the due date, not when it arrives. Record who is qualified.
Do CNG cylinders expire?
Containers carry a service timeframe on their label, and an inspection includes verifying the cylinder is still within it. That life comes from the container standard and the manufacturer's marking rather than from the inspection interval, so read the label on your actual tanks and take manufacturer guidance on retirement. Ask about container records.
Is CNG maintenance cheaper than diesel?
Different rather than uniformly cheaper. You lose the aftertreatment burden that makes refuse hard on diesels, and you gain spark ignition components, gas-specific oil, high-pressure filters, an annual container inspection and facility requirements. Track your own two schedules separately for a year and you will have a real answer rather than a brochure one. Compare both free.
What about our workshop?
Indoor facilities that maintain or park CNG vehicles need methane detection and ventilation designed for a gas that rises rather than sinks. It is a building question with its own codes, and it needs a qualified engineer — but it belongs in the CNG decision from the start, because retrofitting a shop is considerably more disruptive than specifying it up front. Talk through the planning.
Can we run CNG, diesel and electric in one system?
You should — the body, hydraulics, brakes, tires and every inspection obligation are identical across all three, and only the chassis schedule differs. Splitting them across separate tools is what makes mixed fleets feel unmanageable, when the truth is that most of the maintenance programme is shared and only a minority of it is fuel-specific. Run all three together.
Know Which Tank Is Due Without Walking the Yard
FleetRabbit tracks CNG containers as assets in their own right — each with its service timeframe, its own inspection history and the 12-month heavy-duty clock — alongside diesel and electric units on the same records and the same body schedule.
Free for 3 vehicles · no credit card required
