Ford Sigma (Duratec/Zetec-SE)

Ford's small petrol four in 1.25 to 1.6 litre forms, known as Zetec-SE and later as Duratec and Ti-VCT — the engine of two decades of Fiestas, Focuses, Pumas and Ka models, and one of the highest-volume European engines of its period.

It is a belt-driven, port-injected, naturally aspirated design, and that summary carries most of the diagnostic value: none of the direct-injection carbon, high-pressure fuel or wet-belt failures that dominate Ford's later engines apply. What matters is the timing belt interval, because it is an interference design and these cars are now old enough that belt history is often unknown.

The recurring complaints are ordinary ageing: coil packs and leads, the crankshaft position sensor, a sticking thermostat, and vacuum leaks producing lean codes. On the Ti-VCT versions the variable valve timing actuators and their oil control solenoids are the addition worth knowing, and they respond to oil condition more than to age.