Closed loop mode means that the ECU will compare the measured air/fuel ratio to the target air/fuel ratio and automatically increase or decrease fuel delivery accordingly.  In open loop mode, the ECU will still use the oxygen sensor to read air/fuel ratios, but will not adjust fuel delivery as in closed loop mode.  Click on any of the parameters in the image below to jump directly to information on that topic.

Closed Loop High RPM
This is the RPM that must be exceeded before the ECU will enter closed loop mode.  The closed loop minimum temperature must also be exceeded for closed loop operation.  This number should be set to a higher value than the value in Closed Loop Low RPM.

Closed Loop Low RPM
Once the ECU has entered closed loop mode, the RPM must drop below this number before closed loop mode is turned off.

Cold Closed Loop Delay
The minimum time from initial startup before entering closed loop mode when the coolant temperature is below 60 degrees F.

Hot Closed Loop Delay
The minimum time from initial startup before entering closed loop mode when the coolant temperature is above 60 degrees F.

Closed Loop Gain
This parameter affects the amount of fuel correction applied in closed loop mode, based upon the difference of the measured air/fuel ratio from the current target air/fuel ratio.  Large injectors generally require small gain numbers, and vice versa.  An excellent method of determining the best number for your application is to observe the oxygen sensor correction factor in a data log.  If the amount of correction constantly swings from a positive number to a negative number, then your closed loop gain setting should be reduced until this swing is minimized.  If the actual air/fuel ratio never seems to get very close to the target air/fuel ratio, you should increase your closed loop gain setting.  Closed loop operation is most effective when you have the gain number set as high as possible with a minimum amount of “overshoot” in the oxygen sensor correction.

Maximum Closed Loop Step
This parameter defines how large the correction factor can be for each individual step that the ECU calculates and applies fuel correction.  This value is typically left at 9%.

Closed Loop Minimum Temperature
This is the coolant temperature that must be exceeded for closed loop operation to begin.

Closed Loop Enable
Checking this box turns on closed loop operation if the parameters for closed loop operation are met.  If this box is unchecked, the ECU will run in open loop mode.

In addition to the parameters set here, there is one other condition which may inhibit closed loop operation.  If any after-start enrichment fuel is being applied, the ECU will not go into closed loop mode, even if all other parameters have been met.  This is described in greater detail here.

Correction Limits
Allows the user to program the limits for how much oxygen sensor correction can be applied when running in closed loop mode.  Each row corresponds to a row in the target air/fuel ratio table.  Limits for positive correction (add fuel) and negative correction (subtract fuel) can be specified for each row in the target air/fuel ratio table.  The upper rows correspond to “power” levels while the lower rows correspond to “cruise/idle” levels.

It is advisable to keep the negative correction limits in the power levels to a minimum.  In the event that the oxygen sensor would read an artificially rich condition (burning oil, misfiring cylinders, etc.), having a high negative correction limit in the upper rows could allow the ECU to remove a substantial amount of fuel, which at high RPM and power levels could result in damage to the engine.