Summary

00:00 - Edward has asked, what are fuel trims? So what we find is that in most factory cars, the ECU runs in what's referred to as closed loop fuel control during idle and cruise.
00:14 And in closed loop what the ECU is doing is looking at the O2 sensors or oxygen sensors that are fitted to the exhaust and those oxygen sensors give the ECU feedback about the current air/fuel ratio.
00:28 Now for factory cars, emissions is incredibly important so what they want to do while they're idling and cruising is make sure that the air/fuel ratio is constantly cycling very slightly back and forth across that stoichiometric air/fuel ratio.
00:44 And that will give minimal emissions and also ensure that the catalytic converters are working efficiently.
00:50 So the fuel trims are what the ECU uses to correct any error.
00:55 So for example if the curent measured air/fuel ratio is a little bit richer than target, what the fuel trims will do is become negative.
01:03 They'll actually remove a little bit of fuel in order to correct that.
01:06 So we're looking at the fuel trims, we can see how close to optimal our fuel delivery is.