Almost always oil - contaminated, insufficient, or unable to drain - rather than the turbocharger wearing out.

The rotating assembly spins extremely fast on a thin film of oil, so any interruption damages the bearings within seconds.

Contaminated oil carries abrasive particles straight into the bearing clearances, and extended change intervals are the usual origin.

A restricted or blocked oil return is as damaging as a blocked feed, because oil backs up and is forced past the seals into the intake or exhaust.

Carbon builds up inside the feed pipe over time, progressively starving the bearings.

Foreign object damage is the other main cause, and it always means finding where the object came from before fitting a replacement.

Clean or replace the oil feed and return, change the oil and filter, find the root cause, and prime the new unit before starting.

The feed pipe carbonises internally and looks fine externally, so replacing it is the safe default rather than an optional extra.

The return must be clear and correctly routed; a kinked or restricted return causes seal leakage on a brand new unit.

Old oil contains debris from the failed turbocharger, so the oil and filter are changed as part of the job.

Check the intake system for debris and the air filter for damage, and clean the intercooler, which will hold oil from the failure.

Prime the new unit with clean oil and crank without starting where the procedure allows it, so the bearings are not dry on the first start.

Usually turbocharger seals passing oil - but check the crankcase breather first, because pressurisation causes the same symptom.

Worn bearings let the shaft move, and the seals then cannot contain oil, which is drawn into the intake or blown into the exhaust.

Blue smoke on acceleration suggests oil entering the intake side; smoke on the overrun or at idle suggests the turbine side.

Oil pooling in the intercooler or the intake pipework is strong supporting evidence.

A blocked crankcase breather pressurises the engine and forces oil past the turbocharger seals even when they are healthy - checking it is quick and prevents an unnecessary replacement.

A restricted oil return does exactly the same thing, so both should be excluded before condemning the unit.

Movable vanes that change the turbine housing's effective size to give boost across a wider speed range - and they seize with carbon.

A fixed turbine housing is a compromise: sized for low-speed response it restricts at high speed, and sized for high output it is lazy low down.

Variable vanes alter the gas flow angle and effective area, giving good response across the range.

Carbon builds up around the vane mechanism, particularly on vehicles doing short, cool journeys, until the vanes stick.

Symptoms are a loss of power, overboost or underboost fault codes, and sometimes a limp-home mode - all of which look like a failed turbocharger.

A seized mechanism can sometimes be cleaned and freed, and actuators are frequently replaceable separately, so diagnosis before ordering is worth real money.

Core units are professionally remanufactured with a balanced cartridge; workshop repair is not realistic because the rotating assembly must be balanced at very high speed.

A remanufactured unit or an exchange cartridge is a normal, well-supported purchase and arrives balanced and tested.

Balancing is the reason it cannot be done in a general workshop - an unbalanced assembly at those speeds fails immediately and violently.

Cleaning a carboned variable geometry mechanism is a legitimate repair where the bearings are sound.

Electronic and vacuum actuators can usually be replaced separately, and a faulty actuator is often mistaken for a failed turbocharger.

Whichever route is taken, the oil system work is identical and is what determines whether the repair lasts.

Yes - the correct specification, changed on time, matters more than on a naturally aspirated engine because the turbocharger is the least tolerant component.

Oil in a turbocharger bearing is exposed to very high temperatures and shears heavily, so specification and change interval are not negotiable.

Extended intervals are the leading contributor to premature turbocharger failure across fleets.

Manufacturer-approved specifications exist precisely because of these conditions, and an oil meeting only a general grade is not equivalent.

Short journeys are hard on turbocharged engines: the oil never reaches temperature, moisture and fuel accumulate, and carbon builds.

Where duty is short-journey or heavily loaded, shorten the interval rather than following the standard schedule.

On hard-worked turbocharged engines yes - shutting down immediately after heavy load leaves hot oil sitting in the bearing housing, where it carbonises.

When the engine stops, oil circulation stops, but the turbine housing is still very hot.

The residual oil in the bearing housing bakes onto the shaft and the oilways, which restricts the feed for every subsequent start.

A short idle period before shutdown lets circulating oil carry that heat away.

Modern water-cooled bearing housings, and on some vehicles an electric pump that runs after shutdown, reduce the need substantially.

In practice it matters most after sustained heavy work - a long climb, towing, or motorway running straight into a yard - and it is worth making a driver instruction for fleets doing that duty.