Almost always air or starvation rather than a failed pump - low fluid, a suction-side leak, or aerated fluid.

The pump makes noise when it is trying to move fluid that contains air, because air compresses and collapses in the pump.

Check the reservoir level first, and look at the fluid: aerated fluid looks milky or foamy rather than clear.

Air is drawn in on the suction side, so a loose clip or a perished low-pressure hose between reservoir and pump causes it without leaking a drop outward.

A blocked reservoir filter or a collapsed suction hose starves the pump with the same result.

Fitting a new pump without curing the cause gives a new pump that whines - a very common and entirely avoidable repeat visit.

Fluid at the rack gaiters means internal rack seals; at the pump shaft means the pump seal; at a union means a hose or fitting.

The rack's internal seals separate the two pressure chambers and seal the shaft where it leaves the housing. When they fail, fluid collects inside the gaiters.

A gaiter that is heavy and full of fluid is therefore a rack fault, not a gaiter fault.

The pump's shaft seal leaks at the front of the pump, often flinging fluid onto the belt and surrounding components.

High-pressure hoses leak at their crimped ends and at the unions, and they degrade with heat and age.

Clean the area and run the system before diagnosing. Fluid travels, and the wettest point is frequently not the source.

Hydraulic faults are diagnosed with fluid, pressure and leaks; electric faults are diagnosed with a scan tool and fault codes.

A hydraulic system has a pump, reservoir, hoses and fluid, all of which can be inspected, tested with a gauge and repaired conventionally.

An electric system has a motor, a torque sensor and a control unit, and its failures present as a warning lamp and a loss of assistance rather than a leak or a noise.

Electric systems often require calibration after replacement or after any work affecting the steering angle sensor, which needs the correct diagnostic equipment.

They also consume power only on demand, which is a measurable fuel saving over a pump that runs continuously.

For a mixed fleet, the practical consequence is that electric steering work usually has to go where the diagnostic tools are.

Both are commonly remanufactured on an exchange basis, and that is usually the practical route rather than an in-house rebuild.

Remanufactured racks and pumps are supplied tested, with a warranty and a core charge, and are well established for common platforms.

Rebuilding a rack in a general workshop is difficult: seal fitting requires the right tooling and the bore and shaft must be within tolerance.

A pitted or scored rack shaft cannot be sealed against, which is the usual reason a rebuild fails.

Pump rebuild kits exist and are more approachable, though a pump that has run cavitating has often damaged its internals.

Whichever route, flush the system properly - debris from a failed pump circulates and will destroy its replacement.

It matters, and the types are not interchangeable. Different manufacturers specify power steering fluid, automatic transmission fluid or specific synthetics, and the wrong one attacks the seals.

The fluid is both the hydraulic medium and the lubricant for the pump, so its additives matter.

Seal compounds are chosen to suit the specified fluid; a different chemistry can swell or harden them.

Some systems specifically require a mineral fluid and are damaged by synthetic, and some the reverse - the label on the reservoir cap is usually explicit.

Fluid degrades with heat and should be checked for colour and smell; dark or burnt fluid indicates the system has been overheating.

Flush rather than top up when replacing components, and never mix types to make up a level.

Intermittent heaviness usually means the assistance is dropping out - a slipping belt, low fluid, aeration, or on an electric system a thermal cut-back.

A glazed or loose drive belt slips under the high load of parking manoeuvres and grips fine at cruise.

Low fluid or air gives assistance that fades as the pump loses prime, typically worst on full lock.

Holding the steering on full lock builds pressure and heat rapidly and is a habit that damages pumps - assistance genuinely reduces when the fluid is hot.

Electric systems deliberately reduce assistance when the motor or control unit gets hot, which presents as heaviness after repeated manoeuvring.

A binding strut top mount or a seized joint gives heaviness that is mechanical rather than assistance-related, and is worth excluding before condemning the pump.

Bleed the system, set the steering wheel straight, align the wheels, and calibrate the steering angle sensor where one is fitted.

Bleeding is usually done by turning lock to lock with the wheels raised and the engine off or idling, following the manufacturer's method until the fluid clears.

The rack must be centred and the steering wheel fitted straight, or the vehicle tracks correctly with an off-centre wheel.

The track rods are adjusted from the new rack's centred position, so a full alignment follows.

Vehicles with stability control have a steering angle sensor that needs calibrating after the rack or the alignment changes, using a scan tool.

Check the gaiters are correctly seated and clipped. A gaiter that has been left displaced admits water and destroys the new rack.