What to check before buying a used tractor unit

Buying a used tractor unit is one of the biggest financial decisions a haulier makes, often with finance spread over several years. And yet many deals are closed after a glance at the engine and a short drive. At Coatrucks we have been buying and selling commercial vehicles since 1986, and these are the checks that separate a good buy from a problem. You don't need to be a mechanic to carry them out: you need to know where to look and what to ask for.

Engine, gearbox and real mileage

Start the engine from cold, look at the smoke and the fluids, test the gearbox on the road and cross-check the mileage against several sources. These are the checks that uncover the most expensive faults.

Cold start

Ask for the engine to be cold when you arrive. If the truck is already warm, it may be a courtesy, or it may be that from cold it shows what it hides once warmed up. In the first few seconds, watch the smoke:

• Thick, persistent white smoke (not the vapour of the first few seconds in winter): it may be coolant getting into the combustion chamber through the head gasket, the cylinder head or the block. Don't go any further without a diagnosis.
• Blue smoke: oil being burnt, from the turbo, the valve guides or the piston rings.
• Black smoke: on a Euro VI, the particulate filter should not let any black smoke out of the exhaust. If you see it, the filter may be damaged or tampered with.
• Metallic knocking at idle: connecting rods, injectors or timing. Get it investigated before signing anything.

Fluids

• Whitish emulsion on the oil filler cap: oil and coolant may be mixing. On trucks that have done very short trips it can be condensation, but it has to be ruled out.
• Cloudy coolant or oil floating on it: the same problem, seen from the other side.
• Oil level higher than normal: the oil may be diluted with diesel, because of faulty injectors or filter regenerations that don't complete properly.
• Sweet smell at the expansion tank: burnt coolant, a sign of overheating.

Automated gearbox

Almost all recent tractor units have an automated gearbox: Volvo I-Shift, Mercedes-Benz PowerShift, Scania Opticruise, Renault Trucks Optidriver, MAN TipMatic, DAF TraXon or Iveco Hi-Tronix, among others.

None of them should crunch when changing gear. A slow change, a jolt on upshifts or a gearbox that stays in neutral when pulling away uphill points to the clutch or the actuators. On the road test, do a hill start, change gear on an incline and force some downshifts.

Mileage: always cross-check several sources

An odometer can be tampered with. That is why mileage is never taken on trust: it is checked.

1. Tachograph data, explained in the next section.
2. Workshop history and telematics. Most manufacturers offer telematics on their recent models, and a workshop with diagnostic equipment can read operating data from the control units. A seller whose vehicle is in order has no problem showing its records.
3. Consistent physical wear: clutch pedal, steering wheel, seat, floor mats and brakes. If the dashboard shows low mileage and the seat looks as if it has done far more, the problem is not the seat.

Paperwork, ITV and tachograph

Ask for the vehicle's complete paperwork and a tachograph download done in front of you. This is where most inconsistencies turn up, and where many buyers don't even look.

Documents you should see

• Registration certificate and ITV card (the Spanish vehicle inspection card), with any modifications recorded. Check that the unladen weight, the maximum authorised mass and the category match the vehicle in front of you.
• ITV inspection history. For goods vehicles over 3.5 tonnes the inspection is annual up to ten years of age and every six months after that. Long gaps or repeated failures call for an explanation.
• DGT vehicle report (Spain's traffic authority), showing previous owners, seizures, immobilisation orders and other restrictions. If you suspect there is outstanding finance with retention of title, also check the Register of Movable Property (Registro de Bienes Muebles).
• Maintenance history with invoices. It doesn't matter whether it comes from the manufacturer's authorised workshops or a trusted independent garage: what matters is that it is consistent (oil changes at the right intervals, clutch, brakes, timing when due). A truck without a single invoice is not necessarily a bad one, but it forces you to check for yourself what its owner didn't document.
• Transport authorisation. It doesn't go with the truck: it belongs to the company. If you are going to work in for-hire haulage, you will have to add the truck to your own.

The tachograph: which version it has and what its data says

The second version of the smart tachograph is mandatory on vehicles registered from 21 August 2023. In international transport, vehicles over 3.5 tonnes with an analogue tachograph or a first-generation digital one had to replace it by 31 December 2024, and those fitted with the first version of the smart tachograph by August 2025. Since 1 July 2026 the requirement also covers vehicles over 2.5 tonnes engaged in international transport or cabotage.

If the tractor unit is going to do international work, check that its tachograph is the required version. And ask the seller to download the vehicle unit data in front of you (they do it with their company card). What to look at:

• Kilometres recorded by the tachograph against those on the dashboard. If they don't match, there's your answer.
• Speeding events recorded and how often they occur.
• Events and faults recorded by the unit itself, including tampering attempts.
• Calibrations: the tachograph is inspected at an authorised workshop at least every two years, and each inspection is recorded.

A professional seller has no problem downloading the tachograph in front of you. If they refuse, you know what you are being sold.

Owners and type of use

Several owners in a few years is not bad in itself, but it is when combined with high mileage and a thin history. And two tractor units with the same mileage are not equal: one may have run flat motorways at constant speed and the other construction sites and mountain passes. Ask which routes it has done and who has driven it: that is what determines how much working life it has left.

Chassis, fifth wheel, axles and brakes

Badly repaired accident damage on a chassis has no cheap fix: it shows in tyre wear, in the steering and, in the worst case, in the structure. Check the chassis rails, symmetry, fifth wheel, suspension and brakes, ideally with the truck over a pit or on a lift.

The chassis

• Welds on the chassis rails. Manufacturers place strict limits on welding the main chassis rails. A weld bead that isn't from the factory calls for an explanation.
• Fresh paint in specific areas. The original chassis ages evenly; a patch of recent paint usually covers something up.
• Front-end symmetry. Look at the cab head-on from a few metres away: headlights at the same height, bumper aligned, grille centred.
• Equal gaps between the cab and the mudguards on both sides.
• Disturbed joints: chassis bolts or rivets that have been replaced or repainted.

The fifth wheel

This is where the load rests and where the safety of the whole combination is at stake:

• Top plate greased, with no rust from neglect.
• Jaws that lock firmly and release cleanly, with no obvious play.
• Plate wear: deep grooves and polished areas indicate a lot of work.
• If the tractor unit is sold with a semi-trailer, measure the kingpin with a gauge and compare it with the manufacturer's wear limit. The standard sizes are 2 inches (50.8 mm) and 3.5 inches (89 mm).

Axles, suspension and brakes

• Air suspension: bellows with no cracks or patches, levelling valves that respond and no air leaks (listen with the engine off and the system under pressure).
• Shock absorbers: if the truck bounces more than once after you push down hard on one end, they are worn.
• Wheel bearings: with the axle raised, rock the wheel up and down and from side to side. Any play is a warning sign.
• Brakes: discs without heat cracks or deep scoring, and pads above the manufacturer's minimum.
• EBS and ABS warning lights off with the engine running.

AdBlue, SCR and particulate filter

The exhaust aftertreatment system is the most common source of expensive faults on a Euro VI tractor unit. Checking it requires a full electronic diagnosis with professional equipment; looking at the tank is not enough.

What to verify:

• Full electronic scan with commercial vehicle diagnostic equipment, either multi-brand or the manufacturer's own (a generic car code reader won't do). Ask for both active and stored fault codes: codes related to NOx, forced regenerations and AdBlue dosing faults give away a compromised system.
• AdBlue tank: white urea crystals around the filler cap or the connections indicate leaks; cloudy fluid means contaminated or poor-quality AdBlue.
• Particulate filter: the diagnostic equipment shows its soot load and regeneration history. Very frequent regenerations, or frequent forced ones, point to a problem.
• Repair history: if the SCR or the filter has been in the workshop several times in a short period, walk away.

If you want to understand what the standard requires and why these systems are the way they are, we explain it in our guide on Euro 6 for trucks, DGT labels, low-emission zones and Euro 7.

Tyres, cab and equipment

Tyres and the cab rarely kill a deal, but they change what it costs to put the tractor unit to work and what your working day in it will be like.

Tyres

• Brand and range: a set of premium tyres and a heavily worn budget set are not worth the same, even if both technically “have tyres”.
• Even wear: marked wear on the inner or outer edge points to alignment or suspension; wavy wear, to the shock absorbers or bearings.
• Date of manufacture: the four digits of the DOT code on the sidewall give the week and year. A tyre that is many years old can have hardened or cracked rubber even if it still has tread.
• Even tread depth on both sides of each axle.

Cab

• Smell: tobacco, damp, mould or diesel tell you how the truck has really been used.
• Driver's seat: working air suspension, upholstery without tears.
• Climate control: run the air conditioning on cold for five minutes with the engine running. If it doesn't cool properly, it may just need regassing or it may be the compressor.
• Electronics: display, steering wheel controls, cruise control, retarder, and emergency braking and lane assist systems if fitted. Test everything.
• Bunk, if you are going to sleep in the cab: it is not a detail, it's your bed.
• Lights inside and out, indicators, marker lights and beacons.

Equipment you will notice every day

• Retarder or engine brake. It may be a hydraulic retarder (ZF Intarder, Voith or the manufacturer's own, such as Scania's) or an uprated engine brake (such as Volvo's VEB+). Test it going downhill: it should hold the truck back strongly.
• Diesel and AdBlue tanks: capacity to suit your routes, no leaks, and brackets and welds in good condition.
• Hydraulic equipment and PTO, if you need them for a tipper, walking floor or crane: they should work and be properly documented.

Checklist for the viewing

Take this list with you when you view the truck and tick off each point. If something can't be checked, make a note of it: that is information too.

Who you buy from

Besides the truck, it matters who is selling it: a professional seller makes the checks in this guide easy — diagnosis, tachograph download, road test — and documents what they sell.

At Coatrucks we inspect every vehicle in our workshop in Lleida before delivery, and you can ask us for all the information we have on it: mileage, history, owners and repairs. You can browse our current stock of used tractor units, filter by retarder, automatic gearbox or hydraulic equipment and, if you are replacing your tractor unit, sell us your current one. If what you are looking for isn't in stock, we can also bring it in from Europe; we explain the process in our guide on how to import a truck from Europe into Spain.