Do not start the engine — this is the most critical rule
If you realize you have put the wrong fuel in your vehicle, the single most important action is do not start the engine. See the fuel delivery roadside service guide for related situations. If you have already started it, turn it off immediately.\n\nThe damage from wrong fuel is largely preventable if the engine is never started. Wrong fuel sitting in the tank is a contamination problem that requires a drain and flush — a relatively straightforward procedure costing $150-400.\n\nWrong fuel circulated through the fuel system by a running engine is a different situation entirely. Gasoline in a diesel system runs through diesel injectors that are not designed for gasoline lubrication, potentially damaging injectors, injection pumps, and fuel system components. Diesel in a gasoline engine clogs catalytic converters and fouls spark plugs. Total repair costs for a running engine misfuel can reach $1,000-5,000 or more depending on how long the engine ran.
The two most common misfueling scenarios
Two misfueling situations occur most frequently.\n\nGasoline in a diesel vehicle: This is the more common and more damaging scenario. Diesel fuel nozzles are larger than gasoline nozzles — they are specifically designed to not fit standard gasoline filler necks. However, the reverse is not true: a gasoline nozzle fits in a diesel filler neck, making this misfueling easy to do accidentally.\n\nGasoline in a diesel vehicle is dangerous because diesel fuel lubricates the injection system. See the complete guide to diesel fuel delivery situations. Gasoline does not have this lubricating property, and if the engine runs on gasoline, the injection pump and injectors are running without proper lubrication at high pressure.\n\nWrong octane gasoline: Putting regular gasoline in a vehicle that requires premium is a less severe form of misfueling. Modern vehicles with knock sensors can adjust ignition timing to compensate for lower-octane fuel temporarily. This causes no immediate damage but reduces performance and fuel economy. Fill with premium at the next opportunity — no drain is necessary.
What to do after misfueling
If you have misfueled and have not started the engine, the process is straightforward but requires a professional service.\n\nPush or roll the vehicle away from the pump if possible, so you are not blocking the fuel station. Call your roadside assistance program and describe the situation — wrong fuel, engine not started, need a drain and flush. Most major programs have mobile fuel drain services or can tow to a shop that does.\n\nIf you have started the engine and then realized the misfuel, turn it off immediately and follow the same process — call for a tow rather than attempting to drive anywhere. The goal is to stop engine operation on the wrong fuel as quickly as possible to limit damage.\n\nDo not attempt to siphon the fuel yourself unless you have the specific tools and knowledge to do so safely. Fuel is a fire hazard and improper siphoning can introduce air into the system or leave contamination behind.
Cost of wrong fuel repair
The cost of misfueling repair depends entirely on whether the engine was started.\n\nEngine not started: Fuel drain and flush at a shop or via mobile service typically costs $150-400. See general fuel delivery pricing for comparison. The contaminated fuel is removed, the tank is flushed, and fresh correct fuel is added. No mechanical damage occurs in most cases.\n\nEngine started briefly (less than one minute): Additional flushing of fuel lines and possibly filter replacement. Costs typically $300-600. Damage risk is low if the engine ran for only a few seconds.\n\nEngine ran for several minutes or more: Injector and injection system inspection required. Catalytic converter damage possible in gasoline vehicles that ran diesel. Repair costs $500-5,000 depending on the extent of contamination and what components require replacement.\n\nInsurance coverage for misfueling varies by policy. Some comprehensive policies cover misfueling as an accidental damage event; most do not. Check your policy before assuming coverage.