You have noticed you are filling up more often than before, but the routes are the same and your driving style has not changed. Before suspecting a fault, it is worth ruling out external factors that can raise consumption by several liters per 100 km without any engine malfunction. Only if none of those explain the difference should you look for a technical cause.
External Factors That Raise Consumption Without a Fault
Winter alone raises fuel consumption. A cold engine runs on a richer mixture until it reaches operating temperature, and short city trips in winter mean the engine never reaches its optimal temperature. A difference of 1-2 liters per 100 km between summer and winter driving is perfectly normal.
Air conditioning loads the engine and adds 0.5-1.5 liters per 100 km depending on the engine and outside temperature. A roof rack, even empty, disrupts aerodynamics and at highway speeds can add 5-10% to consumption. A roof box makes it even worse.
Tire pressure is the most commonly neglected factor. Just 0.5 bar below the recommended pressure on all four tires increases rolling resistance enough to raise consumption by 3-5%. Check pressure on cold tires once a month. The recommended values are on the sticker inside the driver's door frame.
If you have ruled out all of the above and consumption is still higher than before, the problem is in the engine or one of the systems that affect combustion.
Oxygen Sensor and MAF Sensor, the Engine Running Rich
The two sensors that most often cause increased fuel consumption are the oxygen sensor (lambda probe) and the MAF sensor. Both directly affect how much fuel the ECU injects into the cylinders.
The oxygen sensor measures oxygen in the exhaust gases and the ECU uses that reading to correct the mixture. When the sensor weakens or fails completely, the ECU loses feedback and falls back to pre-programmed values that are intentionally rich. The result is consumption 15-25% above normal. An oxygen sensor typically lasts 100,000-160,000 km, depending on fuel quality and engine condition.
The MAF sensor measures how much air enters the engine. When it gets dirty or weakens, it sends inaccurate data and the ECU doses fuel based on a wrong input. The problem with a failing MAF sensor is that it develops gradually. The car burns a little more every day, and you only notice when you compare consumption with a period several months earlier.
Both problems usually trigger the check engine light, but not always. On older vehicles, the sensor can weaken for years without crossing the threshold that activates a fault code in the ECU memory.
Thermostat Stuck Open
The thermostat regulates the flow of coolant and allows the engine to reach operating temperature quickly (typically 85-95 °C). When the thermostat gets stuck in the open position, coolant circulates through the radiator constantly and the engine warms up slower than it should, or never reaches full operating temperature.
The ECU registers a low engine temperature and keeps the mixture richer, exactly as during a cold start. Except that cold start never ends. In the city the effect may be mild, but in winter temperatures, consumption can jump by 20-30% because the engine practically never exits warm-up mode.
The simplest sign is the temperature gauge on the dashboard not reaching the middle even after 10-15 minutes of driving, or the cabin heater never getting warm enough. More on how the cooling system affects engine operation can be found in the coolant and thermostat guide.
Air Filter, Spark Plugs and Injectors
An air filter that is past its service life restricts airflow into the engine. The ECU partially compensates by reducing fuel, but beyond a certain point the engine simply does not have enough air for efficient combustion and consumption rises. Replacing the air filter is one of the cheapest interventions with a noticeable effect on fuel use.
On gasoline engines, worn spark plugs do not ignite fuel efficiently. Some fuel passes through unburned and the engine burns 10-15% more than it should. Spark plugs are typically replaced every 30,000-60,000 km depending on the type (iridium lasts longer than standard nickel plugs). On vehicles with an LPG system this is even more pronounced because gas requires a stronger spark for ignition.
On diesel engines, the injectors play the key role. Worn or clogged injectors do not atomize fuel into a fine enough spray, combustion is incomplete and consumption rises. Accompanying symptoms usually include rough idle, black smoke from the exhaust and difficult cold starts.
A Dragging Brake and Drivetrain Resistance
A cause that drivers rarely suspect is a brake that does not fully release. A seized caliper slider or a deteriorated brake hose can keep the pad in light contact with the disc even when you are not braking. The engine has to overcome that extra resistance, spending fuel on heating the disc rather than on moving the car.
A simple test: after 15-20 minutes of normal city driving, stop and check the rim temperatures by hand (carefully, do not touch the disc). If one rim is noticeably hotter than the others, that brake is dragging. Another test is free coasting: on a gentle downhill slope in neutral, the car should glide freely. If it noticeably slows down without braking, there is mechanical resistance.
Besides brakes, low pressure in one tire or misaligned wheel geometry after hitting a pothole or a curb can create extra resistance that directly translates into higher fuel consumption.
What the Mechanic Checks During Diagnostics
When none of the external factors explain the rise in consumption, the real answer comes from OBD diagnostics. The mechanic connects a scanner and looks at live data, not just fault codes. The key parameters are fuel trim values (STFT and LTFT), which show how much the ECU deviates from the ideal mixture. If the long-term fuel trim is +15% or more, the engine is constantly adding fuel to compensate for something, and that something is a specific fault that needs to be found.
Coolant temperature readings on the scanner show whether the thermostat is working correctly. If the engine reports 65 °C instead of 90 °C after 10 minutes of driving, the thermostat is stuck open. MAF sensor and oxygen sensor readings directly show whether those sensors are operating within limits or drifting.
Fuel consumption does not drop by magic or through additives. Every extra liter has a concrete cause, and diagnostics is the fastest path to finding it. If you notice your consumption is rising for no obvious reason, get in touch and we will check where the problem is coming from.