If you're running an F23A with an OBD1 ECU, the factory Idle Air Control Valve is one of the main things you need to address during the conversion.
Most F23A applications use a 3-wire IACV, while the common chipped Honda OBD1 ECUs we use are designed to control a 2-wire IACV.
The factory 3-wire valve is not directly compatible with the OBD1 ECU.
It also cannot simply be rewired or modified internally to work like a 2-wire valve. The two systems operate differently.
You have two options.
Option 1: Run Without an IACV
On F23A setups where the factory 3-wire IACV cannot be used, DGTune can configure the calibration so the ECU does not require IACV operation or trigger an IACV Check Engine Light.
The factory 3-wire valve can simply be disconnected.
You will then manually adjust the engine's base idle using the throttle body's idle adjustment.
The downside is that the ECU will no longer have active control over idle speed.
That means you will lose functions such as:
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Automatic cold-idle assistance
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ECU-controlled idle correction
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Idle compensation for electrical loads
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A/C idle compensation where applicable
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Additional idle control when RPM drops quickly
The engine can still run this way, but the idle will be mechanically controlled rather than actively maintained by the ECU.
For a street-driven vehicle, converting to a 2-wire IACV is generally the better solution.
Option 2: Convert to a 2-Wire IACV
The preferred setup is to replace the F23A 3-wire IACV with a compatible Honda 2-wire IACV.
Once properly installed and wired, the OBD1 ECU can control idle normally.
This restores the ECU's ability to:
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Control warm idle
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Assist with cold starts
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Catch RPM when coming to a stop
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Compensate for changing engine loads
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Maintain the requested idle speed in the tune
For most street cars, this is the route we recommend.
You Cannot Use the Original 3-Wire IACV
This is important:
Do not connect the factory F23A 3-wire IACV to an OBD1 ECU and expect it to operate normally.
A 3-wire IACV uses a different control strategy than the older 2-wire Honda valve.
Changing the connector alone does not convert it.
You need an actual 2-wire IACV.
Keeping the Factory F23A Intake Manifold
You do not necessarily need to replace the entire intake manifold.
If you want to keep the F23A manifold, an adapter plate can be used to install a standard Honda 2-wire IACV.
The correct adapter depends on where the IACV is mounted on your manifold.
Rear-Mounted IACV
If your IACV is mounted on the back of the intake manifold:
Front-Mounted IACV
Some applications have the IACV mounted toward the front of the manifold. This setup is less common.
Front-Mount IACV Adapter Plate
You will also need:
2-Wire Honda IACV
A standard compatible Honda 2-wire IACV can be used.
2-Wire IACV Connector/Pigtail
You will also need the matching 2-wire electrical connector.
You do not have to use these exact parts. They are examples of the style of components required.
Using an F22 Intake Manifold
Another common option on an F23A OBD1 conversion is using an F22 intake manifold that is already designed around a compatible 2-wire IACV arrangement.
This can simplify the conversion because you don't have to adapt the original F23A 3-wire valve location.
Either approach can work:
F23A manifold + 2-wire adapter
or
Compatible F22 manifold + 2-wire IACV
The important part is that the final setup uses a proper 2-wire valve that the OBD1 ECU can control.
Wiring the 2-Wire IACV
A Honda 2-wire IACV is simple.
One wire supplies switched ignition power.
The other wire is controlled by the ECU.
The 3-wire system has an additional control circuit, so one of the original wires is no longer needed after the conversion.
99–01 Civic/Accord OBD2B 3-Wire Conversion Wiring
If your F23A swap is installed in a 1999–2001 Civic/Accord using the factory OBD2B 3-wire IACV wiring, the commonly used conversion is as follows.
At the IACV Connector
Remove the factory 3-wire connector and install the replacement 2-wire pigtail.
Orange wire:
No longer used. Insulate it with heat shrink or electrical tape and secure it out of the way.
Yellow/Black factory wire:
Connect to the power wire on the replacement 2-wire IACV connector.
Black/Blue factory wire:
Connect to the control wire on the replacement 2-wire IACV connector.
Solder and heat-shrink the connections whenever possible.
Do not simply twist the wires together.
At the 99–01 Civic/Accord OBD2B ECU Connector
On the factory 99–01 Civic/Accord OBD2B 3-wire IACV arrangement:
Black/Blue wire — move from B6 to B23
The black/blue wire is moved from the original 3-wire IACV control position at B6 to the 2-wire IACV control position at B23.
The orange wire at B15 is no longer used.
Remove it from the connector and insulate it so it cannot short against another circuit.
After the conversion:
B23 controls the 2-wire IACV.
Important: Verify Your Harness Before Repinning
The B6-to-B23 conversion above specifically applies to the common 1999–2001 Civic/Accord OBD2B 3-wire IACV harness arrangement.
Do not assume every F23A swap uses those exact ECU pins or wire colors.
An F23A can be installed into several different chassis and can be using:
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Accord wiring
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Civic wiring
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Modified swap wiring
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Aftermarket conversion harnesses
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Previously modified engine harnesses
If your harness is different, verify the circuit before moving ECU pins.
The important functions are:
One wire → switched 12V power
One wire → OBD1 IACV ECU control
The unused portion of the original 3-wire control system should be safely isolated.
What About the OBD2-to-OBD1 Jumper Harness?
An OBD2-to-OBD1 ECU jumper harness does not automatically convert the IACV.
The jumper allows the OBD1 ECU to connect to the newer vehicle harness, but the physical engine components still need to be compatible with the OBD1 ECU.
That means you still need to either:
Disconnect the 3-wire IACV and run without ECU idle control
or
Physically convert the engine to a 2-wire IACV.
The jumper harness alone does not turn a 3-wire valve into a 2-wire valve.
After Installing the 2-Wire IACV
Once the conversion is complete:
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Check for vacuum leaks.
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Fully warm the engine.
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Verify the throttle cable has some free play.
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Set the mechanical base idle correctly.
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Verify the IACV is connected.
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Reset the ECU.
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Restart the engine and verify that the ECU can control idle normally.
If you're running a DGTune calibration with a modified idle target, the mechanical base idle should also be set appropriately for the requested idle speed in the tune.
See:
How to Properly Set Base Idle on an OBD1 Honda or Acura
Which Option Should You Choose?
For a basic setup, running without an IACV is possible when the calibration has been configured for it.
However, you will be relying entirely on mechanical idle adjustment.
For a street-driven vehicle, we generally recommend converting to the 2-wire IACV.
It gives the OBD1 ECU proper control over idle and provides much better behavior during:
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Cold starts
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Warm-up
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Coming to a stop
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Clutch-in deceleration
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Changing electrical loads
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Normal daily driving
Quick F23A OBD1 IACV Checklist
If you're converting an F23A to OBD1:
Factory 3-wire IACV: Not compatible with normal OBD1 idle control.
Can the 3-wire valve simply be rewired into a 2-wire valve? No.
Can you run without an IACV? Yes, if the tune is configured appropriately, but idle will be manually controlled.
Best street setup: Convert to a proper 2-wire IACV.
Keeping the F23A manifold? Use the appropriate 2-wire IACV adapter.
Alternative: Use a compatible F22 intake manifold with a 2-wire IACV setup.
Using a 99–00 Civic OBD2B harness? Convert the IACV wiring and move the appropriate control circuit from B6 to B23.
Once properly converted, the OBD1 ECU can control the 2-wire IACV normally.
DGTune specializes in Honda and Acura OBD1 ECU tuning, chipped ECUs, ECU socketing, and remote tuning.