Would a faulty ecu cause a gti golf not to run on all four cylinders?
It is very unlikely that an ECU issue would cause the above
problem, and would certainly not affect mk2 and mk3 GTIs due to
their distributor-based ignition system.
Not running on all cylinders is primarily due to a lack of spark
on the affected cylinder.
On earlier vehicles (including GTIs with mechanical injection
and no ECU) the spark may not form at the spark plug due to * a
broken spark plug * a broken spark plug lead (commonly referred to
as a high tension/HT lead) * damaged insulation on an HT lead
(allowing the high voltage to find a path to the vehicle's
electrical ground) * a damaged/worn distributor cap Later
(distributorless) vehicles can have problems with coil-packs (as
seen on the VR6 engines) or coil units (as seen on the 1.8T
Any vehicle with a faulty ECU is most likely to either not start
at all or run in LOS mode. LOS stands for Limited Operation
Strategy and is comparable to 'Safe Mode' on a home computer. In
this mode the car should still run, albeit with reduced power
output. LOS mode is usually triggered by the ECU detecting a fault
condition, faulty sensor or faulty actuator. LOS mode would not
cause the vehicle to run on fewer cylinders.