High fuel consumption
MPG drops noticeably; can be driving conditions, sensor faults, fuel leaks, or brakes dragging.
MEDIUM RISK
Common symptoms
- Strong fuel smell
- Black soot at tailpipe
- Poor throttle response
Cost expectations
Typical costs vary by car and region. Use this as a ballpark.
- DIY / parts: £20–£80
- Garage / labour: £80–£250
Most likely causes (ranked)
Start at the top. Rule out the common, cheap causes before expensive ones.
- Sticking brake caliper — Creates drag; huge MPG hit.
- O2 sensor aging — Fuel trims drift rich.
- MAF/MAP sensor error — Bad load signal = rich fueling.
- Thermostat stuck open — Engine runs cold, stays in enrichment.
- Fuel leak / evap purge fault — Smell + trims; fix for safety.
Quick checks (fast, low-cost)
Fast checks you can do with basic tools before buying parts.
- Check tyre pressures and brake drag (hot wheel).
- Scan for fuel trim readings and O2 sensor activity.
- Check coolant temp reaches normal and stays stable.
- Inspect for fuel smell/leaks under bonnet.
- Check air filter and intake leaks.
Recommended tools
These tools make diagnosis faster and stop you buying random parts.
Related fixes
If your symptoms don’t match perfectly, check these next.
What to do next
- Fix any fuel leak immediately.
- If engine runs cold, replace thermostat.
- If trims are rich, suspect sensors or leaking injector.