A car that started fine and then wouldn't after being left for a week or more is most often a flat battery caused by natural self-discharge or a small parasitic drain. Fuel degradation and rodent damage are less common but worth ruling out after longer lay-ups.
Cars aren't designed to sit completely idle. Even switched off, dozens of small electronic modules — alarms, clocks, keyless entry receivers, trackers — draw a tiny "parasitic" current from the battery around the clock. Over enough days that adds up.
Every lead-acid battery loses a small percentage of its charge per week even with nothing connected. An older or already-weak battery that could just about start the car three weeks ago may simply no longer have enough reserve left after sitting.
Aftermarket alarms, dash cams left plugged in, or a fault in a module that should power down but doesn't, can drain a battery in days rather than weeks. If the car struggles to start after even two or three days sitting, a parasitic drain is more likely than simple self-discharge.
Petrol left sitting for several months can start to degrade and lose some of its volatility, especially in a car stored over winter. This is rarely a factor after a week or two, but becomes more relevant the longer a vehicle has been parked up.
Cars parked for extended periods, particularly near hedges, garages or fields, are occasionally targeted by rodents that chew through wiring insulation. This is uncommon but worth a quick visual check under the bonnet if the car has been sitting somewhere secluded for a long time.
| Likely Cause | Typical UK Cost |
|---|---|
| New battery, fitted | £80 – £180 |
| Parasitic drain diagnosis | £40 – £90 |
| Fuel system clean (degraded fuel) | £60 – £150 |
It varies by battery age and vehicle, but two to four weeks without driving is a common point at which an average battery struggles to start the engine, especially in cold weather.
Petrol can lose some volatility after several months of storage, making cold starts harder, but this is rarely the cause after only a few weeks sitting.
For trips longer than a few weeks, disconnecting the negative terminal or using a trickle charger will prevent the battery from being flat when you return.
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Get My Free Fault ScanLast reviewed 2026-09-08 by the MotorLoom editorial team.