Most modern engines run a closed loop EFI system.
What this means is the exhaust gas is sampled by an O2 sensor and a " rich/lean " signal is sent back to the ECU
If the sample is Rich the ECM leans the mixture out until the preset value of 14.7 is reached
If the sample is Lean the ECM richens the mixture up until the preset value of 14.7 is reached.
The sample is a mixture of all cylnders, so the measured AFR represents an average of all the cylinders.
When the mixture is richened or leaned, all cylinders are richened or leaned by the same amount.

The ecu will calculate a "correction factor" based on the closed loop correction.
This correction factor will be applied to the open loop areas of the map, so both closed loop and open loop operation are adjusted to compensate for variations in air density based on AFR readings.

What stops this working as it should? Dodgy O2 sensor and/or Dirty Injectors.

Here a couple of examples of what will happen when one or more injectors are not performing as they should.

AFR Comparison 1

In this first example the AFR has been corrected to 14.7 but as you can see each cylinder is different. At least 3 injectors are not flowing correctly. Could be 4.

We have no idea which ones are faulty, All will have to be tested & cleaned.

AFR Comparison 2

In this second example the AFR has been corrected to 14.7 but as you can see one cylinder is way lean, 3 cylinders are the same.
We still have no idea as to which injectors are faulty, could be cyl 1, could be 2,3,4, or could be the whole lot.
All will have to be tested & cleaned.

The obvious result of these conditions are an engine not running at its best

• Poor fuel economy
• Reduced Power and Torque
• Reduced Drivability

Because "Dirty Injectors" usually "sneak" up over a period time most people dont actually notice a problem until it gets really bad.

• someone else drives your vehicle and comments on the lack of performance
• you drive another vehicle and realise how poorly your vehicle performs
• it dawns on you that you are using a lot more gas than you used to