P0066 Air Assisted Injector Control Circuit or Circuit Low

P0066 indicates an air-assisted injector control circuit fault or low circuit condition. The air-assist control path is not electrically normal.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not treat this as a fuel injector fault until the air-assisted control circuit is tested.

Symptoms of P0066

Check-engine light on, sometimes with rough or uneven running.
Hesitation, surging, or a noticeable change in fuel economy.
Occasionally hard starting or a smell of unburnt fuel.

Common causes of P0066

A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
Wiring, connector, or fuel-delivery problems that skew the reading.

Useful checks before replacing parts

Measure air-assist control voltage, feed, ground, resistance, and current while checking for short-to-ground or missing-feed conditions.
Inspect connector terminals, vacuum or air-assist valve wiring, fuse supply, harness routing, and module driver output.
Compare circuit behavior with idle quality, cold-start enrichment, fuel trims, and related injector or airflow codes.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.