Fault diagnosis: find the likely cause.
Gearbox, engine, turbo, transmission: 4 questions, and watch the causes narrow down live.
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- Causes ranked by frequency
- Repair or replace: our advice
Every symptom, part by part
39 detailed guides: causes, urgency and when to replace the part. They draw on parts manufacturers' technical documentation and our experience of exchange units since 2014; only a professional can confirm the fault on the vehicle.
Understanding mechanical faults
Gearbox, engine, turbo, transfer case and axle: answers to the questions you ask when a symptom appears.
The most common signs are a noise that disappears when you press the clutch (input shaft bearing), crunching in one specific gear (synchroniser) or a gear that jumps out. Crunching in every gear often comes from the clutch: have that checked first. If the gearbox is at fault, a reconditioned, guaranteed gearbox replaces all the wear parts in one go.
Act without delay. On a dual-clutch gearbox (DSG, EDC), the mechatronics unit or the clutches are often to blame: reading the fault codes points the repair in the right direction. If the gearbox is worn, a reconditioned gearbox supplied with a new dual clutch is the lasting solution.
A dull knock that increases with load, constant blue smoke, heavy oil consumption or loss of compression are signs of internal wear. A reconditioned bare engine lets you keep your ancillaries (turbo, injection, alternator) and costs far less than a complete engine.
Stop immediately and switch off the engine. Driving without oil pressure can destroy the engine within minutes. Check the oil level with the engine off, then have the pump and sensor checked before driving again.
A slight whistle is normal. A new whistle often comes from an air leak in a hose. If the turbo has play, scrapes or lets oil through, it must be replaced: plan an oil change and a check of the oil feed and return so the new turbo lasts.
On a 4x4, check the tyres first: different wear or pressure between front and rear makes the transfer case work constantly. If the noise persists, the internal chain or bearings are worn, and an exchange unit is the most reliable solution.
A howl that increases with speed and changes between accelerating and lifting off signals a worn crown wheel and pinion. Repeated clunking or grinding means you must stop. A reconditioned axle or differential avoids tricky crown wheel and pinion adjustment, and comes with a guarantee.
When the fault is internal, removal costs the same labour either way. A reconditioned part replaces all the wear parts at once and gives you up to 24 months' guarantee, whereas a part-by-part repair leaves the other worn parts in place.
No. It gives the most likely causes based on your answers, so you know what to expect and can prepare your quote. Only a professional can confirm the fault on the vehicle.
