MOT defect file
Fuel filler cap missing or fuel system leaking
Last updated
Quick answer
Fuel is escaping from the tank, a pipe or a hose, or the filler cap is missing or no longer seals. A missing cap counts the same as a leak, so it is worth checking yours is there and tightens properly before the test.
- Typical cost
- £15–£500
- DIY viability
- Possible with care
What you might notice
- A smell of petrol or diesel around the car, strongest after filling up or on a warm day
- A damp patch or drips under the tank, along a pipe run, or in the engine bay after the car has been parked
- A filler cap that no longer clicks tight, has a perished seal, or is simply not there
- Fuel consumption that has drifted worse with no other explanation, and on newer cars an evaporative emissions fault code
What causes it
- A perished or missing cap seal, or a replacement cap of the wrong type that does not seal properly
- Corrosion of the steel tank or the filler neck, particularly on older cars and where the underbody protection has been damaged
- Rubber fuel hoses hardening and cracking with age, or chafing where they pass a bracket or a moving part
- Stone or speed-bump damage to a pipe run, and split seams on a plastic tank after an impact
Advisory or failure?
A fuel system leaking, or a missing or ineffective filler cap, is a major defect — a cap that has gone or no longer seals is graded exactly the same as a leak. A fuel tank, pipe or hose that is insecure is a major defect, and becomes dangerous where the insecurity presents a risk of fire. Leaking excessively, or leaking with a risk of fire, is likewise dangerous. A fuel pipe or hose that is chafing is a minor defect; one that is actually damaged is a major. Two details are worth knowing. A hole or split in the tank or filler neck is failed for leaking even if it sits above the fuel line, so the fact that nothing is dripping today is no defence. And a missing fuel tank or exhaust heat shield that was fitted as original equipment is graded dangerous where it creates a fire risk. Gas systems are stricter still: any part of an LPG, CNG, LNG or hydrogen system found defective is a dangerous defect. If the tester cannot open the filler cap to inspect it, that is a ground for refusing to carry out the test.
Can you fix it yourself?
Possible with care
Replacing a filler cap is genuinely simple and cheap, and it is worth buying the correct part for the car rather than a universal one, since the seal and the venting are part of the design and a poorly sealing cap can set an emissions fault. Anything past the cap needs treating seriously: petrol vapour ignites from a spark or a hot exhaust, so there is no grinding, welding or careless work anywhere near the tank, and a tank that has to come out should be drained and left to air first. Short rubber hose sections can be replaced with correctly rated fuel hose and proper clips — never a garden hose or a jubilee clip pulled tight on hardened rubber — but a corroded metal pipe, a leaking tank seam or any gas system component belongs with a garage. If you can smell fuel and cannot immediately see where it is coming from, stop driving the car.
What it costs
A correct filler cap is £8 to £35 and fits itself, which is the cheapest possible way to clear this failure — check yours before the test rather than after. A short rubber hose and clips is £15 to £40 in parts with half an hour of labour. A corroded metal pipe run is £120 to £300 fitted, depending on how much of the car has to come off to reach it. The top of the band is a replacement tank: £150 to £400 for the part on a common car, with two to four hours of labour because it usually has to be dropped out from underneath, and more where the interior or the rear suspension is in the way. Gas system work sits outside this band and must go to a properly qualified installer.
What happens if you ignore it
A fuel leak near a hot exhaust or a starter motor is one of the few faults on the test with a genuine fire outcome, and a car fire develops far faster than people imagine. Vapour from a missing cap is a fire risk in its own right and puts hydrocarbons straight into the air. Cost escalates in a predictable way too: a weeping pipe union that could have been a hose and two clips becomes a full pipe run, and fuel dripping onto a driveway or the road is an environmental offence as well as a nuisance. Because the more serious versions are graded dangerous, driving the car also means using a vehicle in a dangerous condition, with the fine and penalty points that go with it.
Common questions
- Is a missing fuel filler cap an MOT failure?
- Yes. A missing or ineffective filler cap is a major defect and is graded exactly the same as a fuel leak, so a cap that no longer seals counts too. It is the cheapest failure in this category to avoid entirely: check yours is there and tightens properly before the test rather than after.
- Can I drive without a fuel cap?
- You should not. Vapour escaping from an open filler neck is a fire risk and puts hydrocarbons straight into the air, and on newer cars it will usually set an evaporative emissions fault code as well. A correct replacement cap is £8 to £35 and fits itself, so there is nothing to gain by leaving it.
- Does the MOT tester need to open my fuel cap?
- Yes, the cap is inspected as part of the test, and if the tester cannot open it — a lost key for a locking cap, or one seized in place — that is a ground for refusing to carry out the test at all. Make sure whatever is needed to open the filler goes to the garage with the car.
- Is a leaking fuel tank a dangerous MOT defect?
- It can be. A leak on its own is a major defect, but leaking excessively, or leaking with a risk of fire, is dangerous, as is an insecure tank, pipe or hose that presents a fire risk. A hole or split above the fuel line still fails for leaking, so nothing dripping today is no defence. Any defect in an LPG or other gas system is dangerous.
- How much does it cost to fix a fuel leak?
- A short rubber hose and clips is £15 to £40 in parts with half an hour of labour. A corroded metal pipe run is £120 to £300 fitted, depending on how much has to come off to reach it. A replacement tank is the top of the band at £150 to £400 for the part plus two to four hours, because it usually has to be dropped out from underneath.
Cars this happens to most
Of every model with at least 2,000 tests in the DVSA data, these record fuel filler cap missing or fuel system leaking on the largest share of their MOTs. Across all of them the typical rate is 0.6% of tests.
73 times in 4,726 tests
- Smart (mcc) City1.5%
62 times in 4,194 tests
- Seat Mii1.2%
230 times in 18,638 tests
- Skoda Citigo1.1%
591 times in 52,096 tests
- Volkswagen UP0.9%
1,096 times in 127,183 tests
- Mini Mini0.8%
2,052 times in 246,751 tests
15 times in 2,635 tests
649 times in 119,753 tests
A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.
The warning light behind this
This defect is what a tester writes down. What you see from the driver’s seat is a symbol, and it is worth checking which one, because manufacturers reuse the same shape for more than one system.
Fault codes behind this failure
A failure certificate records that the lamp was lit, not what lit it. If this is what your certificate says, one of these codes is usually stored behind it, and reading it is the difference between a diagnosis and a guess.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.