P0066: Air Assisted Injector Control Circuit or Circuit Low

What it means

The control module detects an electrical fault in the air-assisted injector control circuit, including a circuit-low condition.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

On engines equipped with air-assisted injection, a control circuit operates the valve or solenoid that directs air to help atomize fuel at the injectors. P0066 indicates an electrical fault in that circuit, including a signal below the expected range; it does not by itself identify the exact failed component.

Common causes

Most likely first.

1. Wiring shorted to ground
2. Connector pins corroded - high contact resistance
3. Harness chafing through insulation

Symptoms

• Check-engine light on
• Rough idle
• Hesitation or reduced acceleration
• Engine may run poorly

How to diagnose it

1. Record freeze-frame data and check for related injector or air-assist codes to see whether the circuit fault occurs with rough running.
2. Inspect the air-assist control valve or solenoid connector and harness for chafing, corrosion, loose terminals, or a short to ground.
3. Check the circuit fuse and verify supply and ground at the control device using the wiring diagram.
4. Test circuit continuity and check for a short to ground with the device disconnected, following the manufacturer's procedure.
5. If the wiring and device pass, test the control output using the manufacturer's diagnostic procedure.

Common fixes

• Repair a chafed air-assist control wire or short to ground.
• Clean or repair corroded or loose connector terminals.
• Replace a failed air-assist control valve or solenoid if testing confirms it.
• Repair a confirmed fuse, supply, ground, or module control fault.

Can I keep driving?

If the engine runs roughly or hesitates, avoid hard acceleration and drive only as needed to reach a safe place for diagnosis. Poor fuel atomization can worsen drivability and emissions.