The PD (pump injector) unit is held in the cylinder head by a single side bolt. Over time the seat works itself loose: the mounting slackens, the copper ring no longer seals, and hot combustion gas blows past. First you hear a characteristic "puffing", rhythmic blowing noise from the top of the engine when cold, soot appears under the cam cover, and then the seat surface in the cylinder head erodes. At that point replacing the seal is not enough: the head has to be refurbished or replaced. It affects PD engines between 1998 and 2010, regardless of size.

The light, magnesium cased MQ200 and the MQ250, sized for higher torque, are the most widespread manual gearboxes in the transverse engine VAG models. Both have the same two weak points, and both depend on use.

The internal springs of the flywheel tire, and the two discs knock against each other. The signs are a rattling or knocking from the bellhousing at idle and when releasing the clutch pedal, and vibration through the floor. If the clutch slips without a burnt smell, that also points to the flywheel. It is worth replacing in time: a disintegrated flywheel can destroy the bellhousing too. At a clutch replacement the two go together anyway.

The two weak points of the cooling system are the radiator and the heater matrix. The latter is inside the cabin: if it leaks, you get a sweet antifreeze smell, the windscreen mists up greasily from inside, and damp stays under the passenger side carpet. The damp goes unnoticed for a long time under the floor covering. And replacing the heater matrix means stripping the dashboard.

The PD injectors (on the 1.4, 1.9 and 2.0 TDI alike) are driven by the camshaft itself, against more than 2,000 bar. That wears the lobes and the rockers quickly: first a knocking noise, then a rough idle, hard starting and loss of power. The cause is the wrong oil: the PD engine requires a 5W-40 lubricant to the VW 505.01 specification, with PD approval. A plain synthetic is not enough, and on an everyday 5W-30 the lobes can wear through.

The vanes of the variable geometry turbo are glued together by soot and oil arriving through the EGR. First you get overboost or a lack of boost, then loss of power above 2,000 rpm, and finally limp mode. Caught in time it is enough to remove the turbo, clean it and fit a new actuator rod. Short urban trips and gentle, low rev driving speed up the deposits.

The G62 sensor tells the engine control unit how hot the coolant is. If it is faulty, the control unit thinks a warm engine is cold, and runs it rich: hard warm or cold starting, a rough idle, markedly increased fuel consumption, and a dash temperature gauge that jumps around or never reaches its usual reading. The part is cheap and can be replaced in half an hour: the trouble is that many people do not recognise it and replace an injector or a lambda sensor instead. If the fault persists, the rich mixture can destroy the catalytic converter too.

The G28 sensor drops out as it heats up. While driving the car stops, then starts again after a few minutes. When warm it starts with difficulty or not at all, while cold it starts, so the workshop often cannot reproduce it. The fault code does not always stay stored either.

The Caddy has leaf springs at the back, because it was designed to carry loads, and that is exactly the part that tires from the way it is used. The sign of a tired spring is that the back of the car sits down even unladen (the rear wheel "sinks" into the arch), and a dull clonking or knocking is audible from the rear over bumps. The spring mount and the trailing arm bushes make the same noise. On a heavily loaded fleet car it is effectively a wear item; and a tired rear suspension wears the tyre unevenly too.

The Caddy sliding door is opened many times a day, and the lower roller carries the weight of the door. That is what wears first: the door is hard to start moving, it grinds, it sticks halfway, and finally it "drops". The door gap visibly differs at the top and the bottom. If the mounting of the guide rail works loose, the door presses against the body, and a rub mark is left on the paint. If the roller is not replaced, the hinge and the bodywork are damaged too. On a version with a sliding door on both sides, check both.