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The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability

Regarding the "third" idle air control

As discovered empirically, the FIC mentioned in Page EC-GA-55 of the service manual is the cold-start enrichment mechanism that slightly opens up the throttle plate.

It is not the same as the FICD, which is activated exclusively when the air-conditioning is in use.

The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability

Regarding the "third" idle air control

As discovered empirically, the FIC mentioned in Page EC-GA-55 of the service manual is the cold-start enrichment mechanism that slightly opens up the throttle plate.

It is not the same as the FICD, which is activated exclusively when the air-conditioning is in use.

The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability

The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability

Regarding the "third" idle air control

As discovered empirically, the FIC mentioned in Page EC-GA-55 of the service manual is the cold-start enrichment mechanism that slightly opens up the throttle plate.

It is not the same as the FICD, which is activated exclusively when the air-conditioning is in use.

The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICV is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability

The manual you linked to explains what they're used for.

The AACV is the main idle speed control while the FICD is used to supply additional air to the engine when the AC is turned on.

AACV (Page EC-GA-117)

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via IACV-AAC valve.

FICD (Page EC-GA-122)

When the air conditioner is on, the IACV-FICD solenoid valve supplies additional air to adjust to the increased load.

As for why two IAC's and not one, there could be several reasons for Nissan to set it up like this:

• a bigger IACV wouldn't provide fine enough control (as Fred Wilson points out)
• this is a relatively small engine (1.6 L), so the additional load coming from an AC would result in a large percentage change in air flow which the AACV cannot accommodate by itself.
• parts availability