AdBlue in your fuel tank by mistake?

Yes. It can happen very easily because the fillers are usually next to each other, and many people wrongly believe AdBlue is a fuel additive. It isn’t.

AdBlue is one-third urea (acidic) and two-thirds deionised water. That acid solution gets passed through your engine along with the fuel – with predictable results. Starting the engine makes the problem worse.

It’s also prone to crystallisation, clogging injectors and fuel filters. Fuel filters that have had AdBlue put through them decompose and fall apart, like cardboard soaked in water – and it happens quite fast. The acidic urea and paper particles clog and attack seals, fuel lines and injectors. It needs to be removed quickly, preferably before the engine is turned on.

Provided you act quickly, you’re very unlikely to suffer any serious damage. In the great majority of cases the car is up and running again as soon as we’ve drained the wrong fuel from the system.

It’s best if you haven’t started the car – that means virtually no chance of damage. Even if you have started it, and maybe driven a little before noticing, the chance of lasting damage is slim. If you got home but the car then wouldn’t restart, you’ll still be fine – but get the wrong fuel removed as soon as you can.

The longer the wrong fuel remains in your tank, fuel lines and engine, the more damage it does: it gradually corrodes rubber and plastic seals and tubes, and dissolves lubricating fluids. Petrol cars need petrol and diesel cars need diesel – anything else invites problems.

The small amount of unmixed diesel in your fuel lines, plus the slightly mixed fuel in the tank, can carry you a short distance. But once petrol travels through the low-pressure pump, the fuel lines, the filter, the high-pressure pump, the common rail and finally into the combustion chamber – that’s normally the exact point the engine stops. Petrol in a diesel system removes lubricants and corrodes seals.

Because you likely want to finish your journey and keep using your car, the wrong fuel needs to be removed fully. In the old days you could siphon petrol out with a length of hosepipe, but modern fuel-tank designs make this unfeasible – it takes specialist equipment and expertise to do properly now.

Petrol removes the oily lubrication diesel provides and corrodes rubber and plastic components. Hoses used to empty heavily-misfueled diesel tanks tell the story: drained within an hour, the hose stays clear; left for a day, the hose shows black fluid as rubber from hoses and seals dissolves in the petrol.

An Environment Agency licence is required to transport and store the mixed fuel we remove. It’s securely and safely stored, then ultimately re-refined and returned to the fuel supply chain.

Both are bad news. In the short term petrol in a diesel usually stalls the engine, or runs with reduced power; diesel in a petrol car loses power and runs flat.

Worst cases: petrol in a diesel can eventually damage the high-pressure pump and injectors through lack of lubrication (metal grinding on metal at very high pressure), then cylinder linings. Diesel in a petrol engine can slide past the piston rings into the oil sump, raising oil level and thinning lubrication, risking serious engine failure. Both extremes are rare and need a lot of driving to reach – but the cost of removing the wrong fuel is tiny by comparison.

Usually not. Some dealers and garages treat misfuels as a windfall – we’ve heard of unstarted cars (with only £20 of wrong fuel added) collected from forecourts and given expensive, unnecessary component replacements. Most garages don’t keep the specialist draining equipment we use and find it simpler to call us.

It’s not recommended, though you can try with only a small amount of wrong fuel in a full tank. The engine tends to run poorly, you may trigger dashboard lights that need clearing at a garage, and it may not restart when cold. You risk corroding hoses and seals and damaging fuel pumps – all expensive to fix. Diluting is a gamble; calling us to fix it fully is usually the best and cheapest option.

You’re really not alone – the best estimates suggest over 300,000 people a year in the UK put the wrong fuel in their car. The pumps look similar, you’re distracted, you’re in a rush. We’ll deal with your problem quickly and let you put it in the rear-view mirror.

Neither. They blend very rapidly in the tank into a fairly even mixture. Both come from crude oil; only refining separates them, and once recombined they mix and stay together until re-refined.

Yes, until about the 1970s, as an ad-hoc winter additive in freezing areas. But diesel engines then were low-tech compared with today’s high-pressure systems. Winter Fuel Additive is now added to diesel by refiners from around September, so there’s no need for old home remedies.

The light can signal many things. The wrong fuel throws off the engine’s measurements (air/fuel flow, pressures, timing) and often triggers a fault code that may or may not be accurate. The vast majority of wrong-fuel fault codes are temporary and switch off once the engine is getting the right fuel. It’s very rare for a light to stay on after a drain and restart.

Usually no. Petrol and diesel don’t “sit” in or “condense” inside a filter. During a fuel drain the filter is flushed by pumping the correct fuel through it. A replacement filter costs only a few pounds and takes 6–8 minutes – be wary of inflated bills for it.

On cars older than about 30 years, maybe – if you don’t mind the taste of diesel for a week. But modern cars have anti-siphon devices and saddle-shaped, baffled tanks a hosepipe can’t reach. There’s also the problem of disposing of the mixed fuel legally – we collect it in approved storage and return it to the refinery.

The symptoms are similar – wrong fuel is really just a type of contamination. The cure is the same: fully drain the contaminated fuel from the tank, fuel lines and engine, do any engine-management reset needed, and fill with the correct fuel. We deal with all of these daily.

Check the fuel cap before you fill up – it tells you exactly what fuel the vehicle takes.

A few signs to look for: the vehicle may not start; you may hear spluttering or it may stall; you may see black smoke from the exhaust; the car may overheat; or it may lose power.

Diesel is a fuel oil used in diesel engines; petrol is used in petrol engines. Diesel is thicker and has a higher cetane rating (a measure of ignition quality), igniting more easily under compression – which is why it’s used in diesel engines.