What Actually Happens to a Car That Sits for Six Months

• Standing damage is not one process. It is six unrelated ones running on different clocks — and the fastest of them starts in week one.
• Petrol in a fuel tank is a consumable with a shelf life. E10 typically degrades within about three months; E5 lasts roughly twice as long; diesel around six months before microbial growth becomes a real risk.
• Tyres have two independent clocks: flat spotting from load, and chemical ageing that runs whether or not the car moves. A stored car consumes tyre life without covering a kilometre.
• Starting the engine monthly is not maintenance. On several of these mechanisms it makes things measurably worse.
• Almost everything on this list is prevented by one thing: a proper drive to operating temperature, at intervals, with the brakes worked and the wheels turned.

Ask what happens to a parked car and most answers begin and end with the battery. The battery is simply the first thing you notice, because it is the thing that stops you getting in and driving away.

Underneath it, five other processes are running, and they do not run at the same speed. Some begin within days. Some are invisible until the car is back on the road at speed. Here is the rough sequence.

The rest of this article is that table, explained.

This surprises almost everyone. Petrol is not inert storage — it is a blend of volatile hydrocarbons that evaporate off at different rates, and once the light fractions have gone, what remains is harder to ignite and lower in octane.

In a sealed jerrycan petrol keeps for years. In a vehicle tank, which breathes, it is a different story: E10 often has only about three months of usable life, and E5 roughly twice that. The difference is the ethanol, which is hygroscopic — it pulls water out of humid air.

When the ethanol picks up more water than it can hold in solution, you get phase separation: an ethanol-and-water layer drops to the bottom of the tank, exactly where the fuel pump pickup sits. That layer is corrosive and attacks lines and metal components. The petrol floating above it has lost the ethanol that was carrying part of its octane rating.

Diesel is more robust but not immune. Roughly six months in, the concern shifts to Dieselpest — microorganisms colonising the boundary between diesel and any accumulated water, producing a slime that clogs filters and, in bad cases, disrupts the whole fuel system.

The practical consequence: a car parked in October on a half tank of E10 is running on out-of-specification fuel by January, and by the following summer the tank is a chemistry problem rather than a fuel reserve.

A modern car never fully sleeps. Alarm, telematics, keyless entry and dozens of control units draw current continuously, which is why most cars are no longer reliably startable after three to four weeks.

That part is recoverable. What is not recoverable is what comes next. A lead-acid or AGM battery held in a discharged state sulfates — lead sulfate crystallises on the plates and stops taking part in the chemistry. Recharging a battery that has sat flat for months does not undo it; capacity is permanently gone. On a premium car with an AGM battery and a mandatory registration procedure after replacement, that is €150–400 and a workshop visit.

Behind it sits the much larger number. A hybrid or electric car left at a low state of charge can drift into deep discharge of the traction battery, and the replacement costs are on a different scale: €800–2,600 for a mild hybrid, €2,000–6,000 for a full hybrid, €4,800–10,500 for a plug-in hybrid, and €4,000–12,000 for a full EV pack.

Worth knowing before you park an electrified car for a season: several manufacturers, Nissan and BMW among them, treat deep discharge as a warranty exclusion. It is not a claim you argue over. It is a claim you do not have.

Two separate problems share one system.

Outside, brake discs are usually grey cast iron with no protective coating on the friction faces — that is by design, since a coating would be scrubbed off in the first few stops. Exposed to humidity, they begin forming a rust film within days. A few days of it wipes off on the first few brake applications. Six months of it does not: the corrosion pits the surface, and pitted discs are replaced, not cleaned. Carbon-ceramic discs are the exception here and do not corrode at all — but their pads still bond, and their replacement economics make that a poor consolation.

Which brings up the single most useful piece of advice in this article: do not leave the handbrake engaged on a car you are storing. Pads held against a damp disc for months bond to it. Releasing them then takes surface material with them, or takes the disc. Chock the wheels and leave the handbrake off, or leave an automatic in Park and chock it.

Inside, brake fluid is hygroscopic and absorbs water from the atmosphere through seals and reservoir venting. Water lowers the boiling point, and a fluid that boils under hard use gives you a pedal that goes to the floor. This is why the change interval is time-based rather than mileage-based — the fluid in a car that has not moved in two years is exactly as due as the fluid in one that has covered 40,000 km.

Clock one: flat spotting. The weight of the car sits on the same few square centimetres of each tyre, and the rubber takes a set. After a week it drives out within a few kilometres. After a month it becomes semi-permanent. After several months — particularly at low pressure, particularly under a heavy car — the deformation is structural, normal driving will not resolve it, and the tyre is replaced.

Clock two, and the one almost nobody counts: chemical ageing. Rubber degrades on its own. Plasticisers migrate out of the compound, the material hardens, and wet grip drops measurably. This process is irreversible and entirely independent of mileage. TÜV and ADAC both recommend replacement at around six years from manufacture, with ten years treated as an absolute ceiling.

Put those together and you get the point that catches people out. A stored car's tyres are ageing at exactly the same rate as a daily driver's — but the owner has no odometer cue to prompt them. Tread depth looks perfect. The DOT code on the sidewall, whose last four digits give week and year of manufacture, tells the real story. A collector car with 3 mm of tread and 2014 tyres does not need tread. It needs tyres.

This is the quietest mechanism on the list and the one that produces the most unwelcome surprises.

Rotating shaft seals — crankshaft, camshaft, gearbox input and output, differential — rely on a film of oil to stay pliable and to seal. When the engine stops, gravity does its work: oil drains off the upper surfaces of those seals, and the rubber above the oil line sits dry. Over months it hardens and shrinks slightly.

Nothing appears to be wrong while the car stands. It appears when the engine turns over again, oil pressure comes up, and a seal that no longer conforms starts weeping. A car that was dry when it was parked has a spot under it a fortnight after being recommissioned.

The air conditioning system works the same way. Its compressor shaft seal is lubricated by oil circulating with the refrigerant, and a compressor that never runs is a seal that is never lubricated. Refrigerant seeps out slowly. Six months later the air conditioning blows warm — which is why running the climate control periodically is a maintenance action, not a comfort one.

Foreign owners consistently underestimate this one, because in many countries it does not exist.

German insurers registered around 252,000 marten claims in 2024, paying out €157 million — a record year — at an average of €623 per incident. Martens are drawn to warm engine bays, and they chew: ignition leads, coolant hoses, insulation, wiring looms. A stationary car in an outdoor or semi-rural space is close to ideal habitat, because nothing disturbs it.

There is a coverage trap worth knowing. Comprehensive policies generally pay for the bitten component. Not all of them pay for the consequential damage — so the severed coolant hose is €623 and the engine that overheated because of it may not be covered at all.

Here is the part that makes the rest of the article actionable. Almost every mechanism above is addressed by the same intervention — a real drive, long enough to reach and hold operating temperature.

That last row is why the periodic engine start is worse than doing nothing. Combustion produces water vapour. An engine that reaches full operating temperature evaporates it out of the oil and the exhaust. An engine idled for five minutes in a cold hall produces the water and then stops before it can drive it off, leaving it condensing in the exhaust system and emulsifying into the oil. Do that monthly for a year and you have corroded the exhaust from the inside and diluted the oil, in the name of maintenance.

What actually works is unglamorous: a battery conditioner on the car continuously, tyres at or slightly above placard pressure, the handbrake off, and a drive of twenty to thirty minutes every four to six weeks that reaches operating temperature, uses the brakes properly and runs the climate control.

A car is a machine designed around being used. Almost every failure described here is a system that depends on motion, heat or circulation and is not getting any.

Six months of standing does not destroy a car. It converts a vehicle you can drive away into a vehicle that needs a battery, probably tyres, possibly brakes, a fuel system flush, and a week in a workshop before it is trustworthy at speed. Which is the same thing, expressed as a bill.

If you are storing in Germany, the registration status matters as much as the physical care. Our guide to storing a car in Germany explains deregistered, season-plate and fully-registered storage, and our what standing damage actually costs article prices the failure modes against real storage tiers. For owners who want the custody handled, our managed storage near Berlin includes the conditioner, documented checks and periodic drives that prevent the damage described here.