P0066 — Air Assisted Injector Control Circuit or Circuit Low

OBD2 code P0066 flags a problem with the fuel delivery and mixture control (air assisted injector control circuit or circuit low). Short, gentle driving to a shop is usually possible. Common culprits: weak fuel pump or clogged fuel filter, faulty fuel rail pressure sensor or wiring, clogged or leaking fuel injectors.

powertrain (engine/transmission)

Is it safe to drive?

Prompt diagnosis recommended P0066 can often be driven to a repair shop if the engine runs smoothly, although continuing with the fault may damage emissions components or cause poor drivability. Stop driving if the engine severely stumbles, stalls, misfires, or the check-engine light flashes.

Typical symptoms

The check-engine light illuminates and the scanner reports P0066.
The engine may idle roughly, hesitate during acceleration, or run unevenly.
Hard starting or stalling can occur if the air-assisted injector fault disrupts the air or fuel mixture.
Reduced power or fuel economy may be noticeable on vehicles where the air-assisted injector affects mixture control.
On some vehicles, P0066 is the only noticeable symptom and drivability remains normal.

Common causes

Open or high-resistance wiring in the air-assisted injector control circuit, including a loose or corroded connector.
Short to ground in the air-assisted injector control wiring that pulls the circuit below its specified operating voltage.
Poor ground connection or inadequate power feed to the air-assisted injector control circuit.
Failed air-assisted injector that draws abnormally low current or has an internal electrical fault.
Failed protection device, such as a blown fuse or fusible link, interrupting the injector control circuit.
Faulty PCM output driver, considered only after the wiring, connectors, grounds, and injector have been tested.

How to diagnose with an OBD scanner

Capture P0066 with an OBD scanner Connect an ELM327 adapter with AIScanAuto or another OBD scanner, then read stored, pending, and permanent codes. Record P0066 and any freeze-frame data before clearing codes. AIScanAuto is a generic scan tool and does not replace manufacturer-specific wiring diagrams or active tests.
Inspect the control circuit Examine the air-assisted injector connector, its wiring, and nearby grounds for corrosion, damage, heat chafing, or looseness. Confirm that all terminals are fully seated and that the wiring does not rub against brackets or hot exhaust components.
Test voltage while the circuit is commanded Using the manufacturer wiring diagram, backprobe the injector control terminal and measure DC voltage or the specified waveform with the engine running or the actuator commanded. Compare the result with the manufacturer specification rather than assuming a universal voltage value.
Separate a wiring short from an injector fault With the circuit properly disconnected and the ignition off, check for continuity from the control terminal to ground. If the line remains shorted with the injector unplugged, repair the wiring. If the readings are normal unplugged but abnormal connected, test the air-assisted injector and its power and ground paths.
Check the injector and PCM output last Verify injector resistance, current draw, and operation against service specifications. Check voltage drop across the ground and power connections. Only after those checks pass should the PCM driver be considered as the source of the P0066 low-circuit condition.
Repair and confirm the result Repair the defective connector, ground, wiring, fuse, injector, or PCM output as testing indicates. Clear P0066, complete the manufacturer drive cycle, and confirm that the code does not return while monitoring for hesitation, stalling, or an illuminated check-engine light.

Approximate repair cost

Typical independent-shop ranges are about USD 100-200 for diagnosis and USD 250-700 for a completed wiring repair or air-assisted injector replacement. These figures are estimates and vary by region, labor access, and parts availability.

Common questions

Does P0066 always mean the air-assisted injector has failed?

No. P0066 identifies a low condition in the air-assisted injector control circuit, and the injector may only be one part of the problem. Corroded connectors, an open or shorted wire, a poor ground, or a failed PCM driver can also set the code.

Can I continue driving with P0066 stored?

Usually yes if the engine runs smoothly and the check-engine light is steady, but P0066 should be diagnosed promptly. If the vehicle stalls, loses substantial power, misfires, or shows a flashing check-engine light, stop driving and arrange repairs.

What tests are needed to diagnose P0066?

Start by reading P0066 and its freeze-frame data with an OBD scanner, then inspect and voltage-test the control circuit. Checking the circuit with the injector disconnected helps determine whether the fault is in the injector, wiring, ground, or PCM output.

Will clearing the P0066 code repair the vehicle?

No. Clearing the code only resets the monitor and does not correct the low-voltage condition. P0066 will generally return if the connector, wiring, ground, fuse, injector, or PCM driver remains faulty when the circuit is retested.

When the engine control unit stores trouble code P0066, it means the onboard diagnostics have spotted a problem with the air assisted injector control circuit. Specifically, the module is reporting that circuit voltage has dropped below the acceptable minimum — essentially sitting near ground potential when it should be at a higher signal level. P0066 falls under the powertrain category, which covers anything that affects the engine and transmission management systems.

The air assisted injector system plays a supporting role in fuel delivery. Rather than relying on fuel pressure alone to atomize fuel at the injector, this system uses compressed air to break the fuel into a finer spray, which improves combustion efficiency, especially during cold starts and transient throttle conditions. The ECU constantly watches the electrical circuit that controls this air injection. When the signal stays abnormally low for longer than the programmed threshold — typically across two consecutive drive cycles — the module logs P0066 and illuminates the check engine light. The freeze frame data captured alongside the code is particularly valuable because it records the exact engine speed, load, temperature, and vehicle speed at the moment the fault occurred, giving a technician a precise starting point rather than guessing.

The "circuit low" wording points directly at the problem's nature. Voltage on the air assisted injector control circuit should sit within a defined range. When it drifts down toward ground instead, the usual suspects include a wire with damaged insulation that has shorted to the vehicle frame, a connectorpin that has backed out or corroded, a missing supply voltage feeding the circuit, or a faulty component pulling the entire circuit down. In most cases, the culprit is either wiring damage or a failing pump or sensor upstream in the fuel system that starves the circuit of the pressure or signal it needs.

Common symptoms of P0066

Persistent check engine light, often steady rather than flashing
Noticeable loss of power when the engine is under load or accelerating
Rough idle that feels uneven, sometimes accompanied by hesitation on tip-in
Hard starting or extended cranking time before the engine fires
Fuel economy that drops noticeably compared to normal
Raw fuel smell from the tailpipe in more severe cases

What causes P0066?

A failed air assisted injector or associated control circuit dragging the signal low
The signal wire shorted to ground due to chafed insulation or a pinched harness
Missing or insufficient supply voltage reaching the air assisted injector circuit
A weak fuel pump unable to maintain proper rail pressure
A faulty fuel rail pressure sensor or related wiring with high resistance
Contaminated, diluted, or incorrect fuel grade causing poor atomization
Clogged or leaking fuel injectors disrupting the fuel-air mixture balance

How to diagnose P0066

Hook up a scan tool, confirm that P0066 is the active code, and record the freeze frame data. This snapshot tells you exactly what the engine was doing — rpm, load, coolant temp, vehicle speed — when the fault was triggered, and it should guide every subsequent test.
Pull all stored and pending codes. Related trouble codes frequently expose the underlying cause. While you are at it, search for technical service bulletins tied to your specific make, model, and model year — manufacturers sometimes publish known fixes or ECU updates that resolve the issue without any parts replacement.
Perform a thorough visual inspection of the wiring harness and connectors serving the fuel delivery and air assist systems. Look for rubbed-through insulation, melted wire jackets, corrosion on terminals, or connectors that are unlatched and partially pulled free.
Test for supply voltage at the air assisted injector connector first. Then check the signal circuit for continuity — measure resistance from the signal wire to a known good ground. A reading near zero ohms is a clear confirmation of a short to ground somewhere in the harness.
Measure fuel rail pressure at idle and under manifold vacuum, then compare the results against the factory specification listed in your service manual. Low pressure points toward a failing pump, clogged filter, or a defective pressure regulator.
Monitor short-term and long-term fuel trim values in the live data stream. Positive trims that keep climbing indicate a lean condition — the ECU is trying to add more fuel to compensate for what the injectors or air assist system is not delivering.
Clear the code, complete a full drive cycle that reproduces the freeze frame conditions — the same load, speed, and temperature range — and verify that P0066 does not reappear.

Repair options and costs

The repair sequence should follow the likelihood and cost of each cause, starting with simple electrical checks before moving to component replacement. The most common fixes and their typical parts-and-labor costs at an independent shop are:

Fuel pump replacement: $400–$900
Fuel rail pressure regulator or sensor replacement: $150–$450
Fuel filter replacement: $80–$200
Injector cleaning or replacement: $200–$800

A faulty wire causing the short is usually repaired with a splice or new connector, which typically adds only $50–$100 in labor on top of any diagnosis time.

How much does it cost to fix P0066?

The total bill hinges entirely on which part is actually at fault. Based on typical independent shop labor rates in the United States, P0066 repairs range from as little as $80 for a fuel filter up to $900 for a full fuel pump assembly replacement. Most situations fall somewhere in between, and the actual cost is usually determined once electrical testing rules out wiring problems and pressure testing confirms whether the pump, regulator, or injectors are to blame. Spending time on accurate diagnosis before purchasing parts is the single most effective way to keep the total bill reasonable — the most expensive replacement is rarely the first one you should try.

Can you drive with P0066?

Drive with caution. P0066 indicates a fault that disrupts the fuel mixture or delivery circuit, and depending on how severely the system is compromised, you may experience hesitation, stalling, or rough running — all of which become safety concerns in traffic or at intersections. In more advanced cases, a severely rich fuel mixture can damage the catalytic converter over time. Short trips to a repair facility are acceptable, but extended highway driving is not recommended until the code has been diagnosed and the underlying problem corrected.