SENSORS
If the ECU has been in the car, so have the sensors. Sensors are the “eyes and ears” of the whole system and give the basic input to the ECU.
But as with all electric appliances, sensors give out to time and material fatigue. Especially if they’re installed in places that are susceptible to heat, moist, oil, filth or else.
Replacement in most cars is simple and basically means taking of a couple of screws or nuts, disconnecting, taking out the old one, putting in the new one and re-connecting. This, of course, depends on the accessibility of the sensor.
Main problem here, in most cases, is making a quality diagnosis. Of course, there is the OBD diagnostic tool, but a faulty sensor can be hard to pin-point and can easily lead to bigger, unneeded expenses.
Even worse, they can, sometimes, start and stop working in irregular patterns, giving false code readings.
One of the trickier car electric problems where looking twice and double-checking is worth gold.
Sensors are also one of the main causes of the dreaded check engine problem.