A fault that causes the warning light to stay on permanently. It usually indicates that the fault is occurring now
and is caused by something physically broken, like broken wire, actuator or sensor.
Soft faults can be caused in three ways,
Caused when then ECU receives information from sensors that cannot possibly be true. For example the car may
have two sensors that record vehicle speed. One may say that the car is moving at 50kph while the other recording 80kph. It's unlikely
that both are correct so the ECU will turn the warning light on while the fault is occurring and record a fault code.
Are caused by something external such as a flat battery. The ECU records that there has been a break in the power supply,
turns the warning light on and sets a fault code. These can normally only be reset by a Scan Tool.
Can be Hard or Soft faults. They can occur on regular occasions or may be months apart. They can be the difficult to
trace unless the fault is occurring while the vehicle is in the workshop. But with good equipment, diagnostic procedures and patience these
faults are always repairable.
Fault Code Hierarchy
Fault codes also have a hierarchy, the more serious the manufacturer deems the fault can also be indicated by how long the warning light stays on.
If the light stay on all the time then the fault is usually serious or it is a hard fault. If the light stays on until the car is switched off, but not when restarted,
then a code will have been logged, but the fault is no longer present. And occasionally the warning light will only flash on when driving.
All faults occur for a reason, catching the cause early can often prevent serious damage later.