January morning at DIRFT, 05:20, headtorch, breath fogging. The curtainsider had been sitting since Friday afternoon and nobody had plugged it in. First turn of the key: nothing. Second turn: a sound like the engine was thinking about it very hard and then deciding against. It took me twenty minutes and jump leads from a neighbouring unit before I was moving. Missed my slot at the first drop by an hour.
Cold-weather failures in HGVs aren't complicated. They're usually one of four things: the AdBlue has frozen, the diesel has waxed, the battery hasn't got the cold-cranking amps to turn the engine over, or the air brake system hasn't bled properly and you've got moisture in the lines. All of them are preventable. None of them are pleasant at 05:30 in a yard in Coventry when it's -9 outside and the job sheet doesn't have any slack in it.
AdBlue: what happens at -11°C
AdBlue is a 32.5% aqueous urea solution — the standard is set by ISO 22241 — and it has a precise freeze point of -11°C. Not approximately -11°C. That's the chemistry of the specific concentration: below that temperature, it solidifies.
Most modern HGVs with SCR (Selective Catalytic Reduction) emissions systems have an integrated tank heating element. Once the engine is running, the heater starts working on the frozen AdBlue and the system typically recovers. The problem is the start — if the AdBlue supply line between the tank and the dosing unit is frozen and the engine's already running, you'll get an SCR fault on the dash. Depending on the vehicle, you might also trigger limp mode: a speed restriction that kicks in after a warning period to force the driver to address the fault before it becomes a shutdown.
Frozen AdBlue that thaws normally is fine. The fluid doesn't degrade through freeze-thaw cycles under normal conditions. What it doesn't do is clear a fault code by itself once the code is set — you may need to clear it manually or wait for the system to reset, depending on the manufacturer.
What to do: if you know the vehicle has been standing outside in sub-zero temperatures and AdBlue is a concern, start the engine and give it 15 to 20 minutes before putting it under load. Let the heating system work. Don't start it, check the dash, see an SCR warning, and immediately call it broken. Give it time first.
Diesel: the wax you can't see until it's a problem
Diesel fuel contains paraffin wax. In normal temperatures the wax stays dissolved. As the temperature drops, it starts to crystallise — this is the Cloud Point, when the fuel first goes cloudy. Keep dropping and you hit the Cold Filter Plugging Point (CFPP), which is where the wax crystals are large enough to block the fuel filter. The engine starts starving for fuel. Power drops. Eventually it won't run.
UK pump diesel is blended seasonally. Winter-grade fuel — typically supplied from around October through March — is blended with a lower CFPP specifically to handle colder conditions. Summer-grade fuel left in the tank from autumn can cause problems in a hard freeze that winter-grade would handle.
Anti-gelling additives (often called diesel conditioner or winter diesel treatment) can be added to the tank when you're heading into a cold snap. They lower the CFPP of whatever's in the tank. Most truck stops sell them. They don't cure a filter that's already blocked — they're preventive, not remedial. If the filter's already waxed, you need to change the filter and warm the system before adding anything.
Older vehicles — pre-Euro VI, older fuel system designs — are more susceptible than modern engines with better fuel line insulation and filter heating. But even a modern DAF or Scania can suffer in an extended deep freeze if it's been standing outside.
Signs of waxed diesel: engine starts fine and then runs progressively worse as the fuel filter gets more restricted. Power loss, hunting idle, difficult restart after a brief stop. Not a sudden cutout — gradual degradation. If this is happening in cold weather, get to a warm workshop and change the filter.
Battery: the silent failure
Cold weather reduces battery capacity and increases cranking load simultaneously. A battery that turns the engine over without complaint in September may not have the cold-cranking amps to do the same job at -8°C in January. And the older the battery, the worse this effect becomes.
HGV starter batteries typically run at 24V (two 12V batteries in series). A fully charged 12V battery reads around 12.6V. Below 12V, you've got a problem. Below 11.5V with cold temperatures on top, you're probably not starting the vehicle without a jump.
Check your battery voltage before a hard winter if you haven't already. If the battery is more than three or four years old and showing any signs of slow cranking in normal temperatures, replace it before winter, not during. The cost of a battery is a fraction of the cost of a breakdown call-out at 05:30.
If you're regularly parking in a yard with facilities, use them. A block heater keeps the coolant warm and significantly reduces cold-start load. Some operators run trickle chargers on vehicles standing for extended weekend periods. Both reduce the cold-weather battery risk substantially.
Air brakes and moisture
Air brake systems on HGVs include dryers to remove moisture from the compressed air. In cold weather, if the dryer isn't working properly — or if the system's been drawing in humid air faster than the dryer can handle — moisture gets into the lines and chambers. At -5°C or below, that moisture freezes. Frozen moisture in a brake actuator or line means reduced or no braking in the affected wheel position.
The walkaround check should include draining the air tanks — most vehicles have a drain valve at the bottom of each tank. Do this daily in cold weather; don't skip it because it's dark and the valve is at the back of the chassis and your hands are cold. Water in the air system isn't obvious until it freezes.
If you see more than a small amount of water when you drain the tanks, the air dryer needs attention. That's a maintenance conversation, not a shrug-and-move-on situation. A driver who reports it in the defect log and a TM who actions it is how you avoid an OCRS problem later when DVSA finds an air brake fault at the roadside.
The pre-start cold-weather routine
This is what I do now when I'm taking a vehicle that's been outside in freezing conditions:
- Check tyres while they're cold — tyre pressures drop roughly 1 PSI per 10°C temperature drop. A tyre that was at correct pressure in September might be notably low in January. Check against the manufacturer's specification.
- Drain the air tanks and look at what comes out
- Start the engine and give it time to warm before checking the dash carefully — AdBlue warnings, air pressure, coolant temp
- Let the air system build to operating pressure and check the brake performance on foot before moving
- If the vehicle has been standing three or more days in sub-zero temperatures, give yourself extra time before the first move
The kit in the cab
In January I keep a few things in the cab that I don't bother with the rest of the year. A good quality jump lead set — not the thin ones from a pound shop. A can of proprietary diesel anti-gelling treatment in case I end up at a yard that's only got summer-grade fuel in the pump. A small battery-powered torch with fresh batteries because the headtorch I usually use will go flat faster in the cold. And a warm pair of overalls for doing the walkaround in a January yard at 05:30 when it's -8 and there's a strong easterly coming across Northamptonshire.
None of this replaces good pre-winter vehicle prep. If the PMI is due before the first hard freeze, do it then and flag anything battery or air system related. Cold weather doesn't create faults — it reveals ones that were developing all autumn.
If you're managing a fleet over winter and tracking PMI dates and driver defect reports in a spreadsheet, ShiftOwt keeps compliance records in one place so cold-weather maintenance flags don't fall through the cracks — fleet pricing on request.