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Higher fuel consumption: causes and how it's checked

By Altay Hasanli, auto electrician

Short answer

If consumption has risen noticeably under the same driving conditions, the engine control unit is usually running the mixture too rich: the oxygen or temperature sensor is misreporting, an injector is delivering too much fuel, or the ignition is misfiring. The Check Engine light may not even be on. The cause is found by diagnosing live data, not by swapping parts on a guess. First check the tyre pressure and the air filter: they affect consumption too.

The most common causes

  1. 1

    Oxygen sensor and air-flow sensor

    A worn oxygen (lambda) sensor responds late or sends a wrong signal, and the control unit enriches the mixture. Consumption creeps up, sometimes with black exhaust smoke and a smell of fuel.

    Fix: Computer diagnostics

  2. 2

    Dirty or leaking injectors

    If an injector sprays poorly or drips after closing, the fuel burns badly, the engine runs rough and consumption rises. Injectors are cleaned and tested on a bench.

    Fix: Injector cleaning

  3. 3

    Ignition: plugs and coils

    A worn plug or a weak coil can't ignite the mixture fully. Unburnt fuel goes out the exhaust, power drops and consumption rises. Such misfires show up clearly on an oscilloscope.

    Fix: Oscilloscope diagnostics

  4. 4

    Coolant temperature sensor

    If the sensor reports the engine as permanently cold, the control unit adds fuel as for a cold start even on a warm engine. Consumption rises and the fan may behave oddly.

    Fix: Computer diagnostics

What you can check yourself

  1. Check the tyre pressure: low pressure raises consumption.
  2. See whether the air filter is clogged.
  3. Calculate consumption several times, filling the tank to full at the same station each time: one measurement isn't reliable.
  4. Note when it went up: in town, on the motorway, on a cold engine, with the A/C on, or after changing fuel station.
  5. Tell us about any rough running, hard starting or black exhaust smoke.

When to stop driving

  • Black exhaust smoke, a smell of raw fuel or a flashing Check Engine light mean misfires: the catalytic converter can be damaged, so don't drive far.
  • If you smell fuel in the cabin or see drips under the car, that's a leak: there's a fire risk, so don't start the engine.

How we find the cause

  1. 1We read fault codes and live data with a scan tool: the oxygen sensor, fuel trims and coolant temperature.
  2. 2We check the oxygen sensor's signal speed and the injector pulses with an oscilloscope.
  3. 3We check the plugs, ignition coils and misfires.
  4. 4If needed, we test and clean the injectors on a bench.

This guide is general information: one symptom can have several causes, and only a diagnostic shows the exact one.

Services and prices

Frequently asked questions

How much of a rise counts as a problem?

The figure depends on the car, so compare it with your own earlier average. A noticeable rise on the same roads and in the same conditions is reason enough for diagnostics.

If Check Engine isn't lit, is everything fine?

No. Readings can drift from normal without the light coming on until a fault code is stored. Live-data diagnostics sees that drift.

What do diagnostics and injector cleaning cost?

Computer diagnostics starts at 20 AZN and injector cleaning at 40 AZN. Once the cause is found, we agree the final price with you upfront.

Other problems

Describe the symptom and we'll tell you what to do

Call, or send photos and videos of the problem on WhatsApp: we'll tell you upfront whether the car needs to come in.

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