P0066 — Air Assisted Injector Control Circuit or Circuit Low
P0066 is the standardized powertrain DTC for “Air Assisted Injector Control Circuit or Circuit Low.” The code identifies the monitored condition; it does not by itself prove which part failed.
What P0066 means
P0066 is the standardized powertrain DTC for “Air Assisted Injector Control Circuit or Circuit Low.” The code identifies the monitored condition; it does not by itself prove which part failed.
How to interpret the code wording
“Circuit Low” means the module saw voltage or a signal below its calibrated range. Depending on circuit design, likely test paths include a short to ground, missing reference/supply voltage, excessive resistance, poor terminal contact, or a failed sensor/actuator. Verify the circuit diagram and measure the circuit before replacing the named component.
Possible symptoms
- Cylinder misfire
- Rough idle
- Hard start
- Fuel odor or poor economy
- Check Engine Light
Causes to test
These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.
- Injector electrical open/short or poor connector contact
- Injector mechanically clogged or leaking
- Driver/control-circuit fault
- Fuel-pressure problem
- Mechanical compression issue mistaken for injector imbalance
Step-by-step diagnostic workflow
- Identify the cylinder and save misfire/fuel-trim data.
- Inspect injector connector and harness; verify supply and control with the correct test method.
- Use injector balance, current-ramp, or contribution testing where specified.
- Confirm fuel pressure and mechanical compression if the cylinder remains weak.
- Perform circuit or mechanical tests specified by service information; repair the confirmed root cause.
- Clear codes only after repair, then complete an appropriate drive cycle and verify the code and symptoms do not return.
Scan data worth checking
- Misfire counters
- Injector command/pulse width
- Fuel trims
- Fuel/rail pressure
- Cylinder contribution data where supported
Severity and drivability
Moderate to high if the engine is misfiring. A leaking injector can also damage the catalyst or dilute engine oil.
Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.
Common diagnostic mistakes
- Using a test light that can overload a PCM injector driver
- Replacing an injector without checking compression
- Ignoring a leaking injector because electrical resistance tests normally
Related generic codes
Source and accuracy notes
Generic DTC structure and standardized definitions are governed by SAE J2012/J2012DA. Actual enable criteria, thresholds, circuit pinouts, test values, software conditions, and repair procedures vary by vehicle. Use current OEM service information for the exact year/engine/calibration and check applicable manufacturer technical service bulletins. See the site methodology and primary-reference policy.