The most common causes of a sudden MPG drop are a faulty oxygen (O2) sensor, dirty MAF sensor, sticking brake caliper, low tyre pressures, or a clogged air filter. A 10–20% drop usually has a single fault behind it; a gradual long-term drop is often tyres + driving style + winter fuel blends.
A sudden 10–20% drop in fuel economy almost always points to a sensor or mechanical fault. Here's the order a UK mechanic checks them.
Tyre pressures (free check)
Underinflated by 10 PSI = roughly 5% worse MPG. Check cold, against the door-pillar sticker. Single biggest free MPG win.
O2 (lambda) sensor
Old or contaminated O2 sensors cause the engine to run rich. Symptom: sudden MPG drop, sometimes EML, no other obvious fault. Replacement: £80–£200.
MAF sensor
Dirty mass-airflow sensor reports wrong air values, fuel trim goes wrong. Often cleanable with MAF spray (£8) before replacement (£100–£250).
Sticking brake caliper
If a caliper drags, the engine is fighting brake friction every mile. Symptom: hot wheel after a short drive, slight pull to one side. Fix: £120–£280 per side.
Clogged air filter, EGR valve, DPF (diesels)
All restrict airflow and force the engine to work harder. Service-item air filter £15–£30. EGR clean £180–£400. DPF regen/clean £250–£900.
Driving style + cold weather
Winter MPG is naturally 10–15% lower (cold engine, dense air, heater load, winter fuel blend). Aggressive acceleration adds 20%+ to fuel use vs smooth driving.
Will a service improve my MPG? — Yes — fresh air filter, oil and spark plugs typically restore 3–8% MPG on a tired engine.
Why does my car use more fuel in winter? — Cold engines run rich until warm, dense air needs more fuel, and winter diesel/petrol blends have slightly less energy. 10–15% drop is normal.
More detail on: Why is my car using more fuel than usual?
The short answer above covers the usual case. Below is the extra detail a UK driver normally needs to act on it — how to narrow the fault down, what it should cost, and when it is worth paying a garage rather than carrying on. This is general guidance from a working mechanic, not a substitute for someone putting your own car on a ramp.
How to describe the problem so you get a straight answer
Mechanics diagnose faster when they get facts rather than conclusions. Give the make, model, engine size, fuel type, year and mileage; say exactly when the fault happens (cold start, motorway speed, braking, turning, first five minutes); say what changed just before it started; and list any codes, warning lights or recent work. A three-line description in that shape is worth more than a paragraph of theory.
What to do before you spend money
Run a free MOT history check to see whether the same item has been advisoried before — that history often explains a fault in one line. Check whether an open safety recall covers the component, because that repair is free at a franchised dealer. Then get the fault properly diagnosed rather than replacing parts in order of price.
Safe to keep driving?
Treat anything affecting brakes, steering, tyres, seat belts or visibility as stop-now, no matter how well the car appears to be driving. Overheating and low oil pressure are also stop-now: the damage curve on both is measured in minutes, not miles. Most other faults will let you drive gently to a garage, but if the car is in limp mode, losing fluid, or the fault changes how it steers or stops, get it recovered instead. Recovery is cheaper than a tow plus a new engine.
What Bob would check first
Start with the cheap, common and visible: fluid levels and their condition, tyre pressures and wear pattern, battery voltage and terminal condition, and every connector and vacuum pipe in the area of the fault. A surprising share of electrical gremlins are a corroded earth or a chafed loom, and a surprising share of running faults are an air leak downstream of the sensor doing the complaining. Only after that does anything get unbolted.
When it is worth going to a main dealer
Three situations justify main-dealer money: an open safety recall (free), a live manufacturer warranty or goodwill claim, and coding or programming work that needs manufacturer-level software. Everything else — servicing, brakes, suspension, clutches, cambelts, diagnostics — an experienced independent will do to the same standard for a good deal less, and using an independent does not invalidate your warranty under UK block exemption rules as long as the correct parts and schedule are used.
Get an answer on your own car
If you want this looked at properly for your exact vehicle, send the details and Bob will write back — make, model, engine, mileage, when the fault happens and any codes you have. You get a written reply from a qualified UK mechanic with 32 years on the tools, not a chatbot, and your money back if he cannot help.
Ask a mechanic about your car · Free MOT history check · Free recall check · Fault code library