F20A 's service schedule should incl checking throttle*sensor 's wear
 

every 60,000 km ; *'s resistance betw the 2 pins connected to yellow+white & red+black wires ( on *'s plug ) ↑ with wear ( as the electrical contact areas ↓, the ampere flowing through ↓, so resistance ↑ ). A usable * ( @ rest ) can have just 411 ohm but a worn * ( causing idle rpm to fluctuate betw 1400 & 600 ) can have 1110 ohm. ECU interprets this higher resistance as throttle plate having turned >20º, & so injects more petrol, so (i) spark plugs foul (ii) bottom of the idle air screw ( on throttle body ) gets much soot, so less & less air can flow past, so idle rpm keeps dropping.
Meters' panel does not show, & very few users connect a volt meter to, O2 sensor's output voltage ( -ve during idling, if sensor is not hot enough ), so nearly no user can notice too much petrol is injected.

F20A 's service schedule should incl checking throttle*sensor 's wear
 

I have removed * altogether, after I connected a 380 ( any value between 375 & 410 can do ) ohm resistor to these 2 wires, so ECU always detects just 380 ohm i.e. ECU presumes throttle is closed, so ECU will never ground injectors longer to inject more petrol. Then engine works fine since 1-20 : torque is already 2x original after I corrected ~15 design flaws, so fuel enrichment by ECU is no longer necessary.