If your OBD-II scanner displays P0066, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a low-voltage condition or another electrical fault in the air-assisted injector control circuit.
The generic SAE description for P0066 is:
Air Assisted Injector Control Circuit or Circuit Low
Depending on the vehicle manufacturer and scan tool, the description may appear as:
- Air Assisted Injector Control Circuit Low
- Air Assist Injector Control Circuit Low
- Air-Assisted Fuel Injector Control Circuit Low
- Air Injector Control Solenoid Circuit Low
An air-assisted injector system introduces controlled airflow near the fuel injectors or intake ports to improve fuel atomization. This is especially useful during cold starts, idle, engine warm-up, and other operating conditions where liquid fuel may not mix efficiently with the incoming air.
P0066 means the ECM has detected voltage below the calibrated range in the control circuit. Depending on the system design, this may be caused by excessive current draw, a wire shorted to ground, a shorted solenoid coil, damaged wiring, or a failed air-assist control valve.
Common causes include:
- Air-assist control wire shorted to ground
- Internally shorted control solenoid
- Failed air-assisted injector valve
- Melted or chafed wiring
- Corroded electrical connector
- Water intrusion
- Incorrect replacement component
- Weak battery or charging voltage
- ECM control-driver failure
P0066 may cause rough cold starts, hesitation, unstable idle, increased emissions, poor fuel economy, or stalling. Some vehicles may show no noticeable symptoms beyond the Check Engine Light.
For additional diagnostic information, visit:
- Complete OBD-II Code Library: https://my.prostreetonline.com/dtc-obdii/
- Body B-Code Library: https://my.prostreetonline.com/dtc-doctor/b-code-obd-ii-codes/
- Chassis C-Code Library: https://my.prostreetonline.com/dtc-doctor/c-code-obd-ii-codes/
P0066 Quick Facts
| Item | Information |
|---|---|
| OBD-II Code | P0066 |
| Description | Air Assisted Injector Control Circuit or Circuit Low |
| Category | Powertrain |
| System | Fuel and Air Metering |
| Main Component | Air-Assisted Injector Control Valve or Solenoid |
| Fault Type | Low Voltage or Excessive Current |
| Severity | Moderate |
| Safe to Drive? | Usually, with caution |
| Typical Repair Cost | $75–900 |
What Does P0066 Mean?
P0066 means the ECM has measured lower-than-expected voltage in the electrical circuit controlling the air-assisted injector system.
On many systems, the ECM controls an air-assist solenoid or valve by switching the circuit to ground. The ECM expects circuit voltage and current to remain within a calibrated range whenever the valve is commanded on or off.
A circuit-low condition may occur when:
- The control wire is shorted to ground
- The solenoid coil draws excessive current
- The control valve is internally shorted
- Two damaged wires contact each other
- Water or corrosion creates an unintended ground path
- The ECM driver remains grounded
- Charging-system voltage is unusually low
P0066 is primarily an electrical circuit code. However, mechanical problems in the valve or solenoid can sometimes increase current draw enough to trigger it.
What Is an Air-Assisted Injector System?
An air-assisted injector system directs additional airflow around the fuel injector nozzle or into the intake port.
This extra airflow helps break liquid fuel into smaller droplets before it enters the combustion chamber.
Improved fuel atomization can provide:
- Smoother cold starts
- More stable idle
- Cleaner combustion
- Reduced hydrocarbon emissions
- Better low-speed throttle response
- Less fuel condensation inside the intake
- Faster catalyst warm-up
Depending on the vehicle, the system may use:
- An electrically operated solenoid
- A vacuum-controlled valve
- Air galleries inside the intake manifold
- Small passages near the injectors
- A dedicated airflow control valve
- Intake manifold vacuum as the air source
This system should not be confused with secondary air injection, which usually pumps air into the exhaust system. Air-assisted injectors introduce air near the fuel injectors or intake ports instead.
Why Is Air Added Near the Fuel Injectors?
Fuel burns most effectively when it is finely atomized and evenly mixed with air.
During a cold start, fuel may:
- Condense on cold intake surfaces
- Form large droplets
- Burn incompletely
- Create an unstable idle
- Produce higher hydrocarbon emissions
- Require additional enrichment
The air-assisted injector system improves the spray pattern and helps the fuel remain suspended in the incoming air.
Once the engine warms up, fuel atomization improves naturally and the ECM may reduce or discontinue air-assist operation.
How the Air-Assisted Injector Circuit Works
A typical air-assisted injector control circuit may include:
- Battery or ignition voltage
- Fuse
- Relay or power feed
- Air-assist control solenoid
- Air-control valve
- ECM-controlled ground circuit
- Wiring harness
- Electrical connector
- Vacuum or airflow hoses
- Intake manifold passages
On many vehicles, one side of the solenoid receives battery voltage when the ignition is on.
The ECM controls the other side by switching the ground circuit through an internal transistorized driver.
The ECM may monitor:
- Control-circuit voltage
- Solenoid current
- Coil resistance
- Commanded duty cycle
- Intake pressure changes
- Engine-speed response
- Oxygen sensor feedback
- Fuel-trim response
If circuit voltage drops below the expected level, the ECM may store P0066.
What Does “Circuit Low” Mean?
“Circuit low” normally means the ECM is seeing less voltage than expected in the monitored control circuit.
This does not necessarily mean the battery itself is low.
Possible circuit-low conditions include:
- Short to ground
- Internally shorted solenoid
- Excessive current draw
- Damaged insulation
- Crossed wiring
- Corroded connector terminals
- Stuck ECM ground driver
For example, if the control wire rubs through and contacts the engine or body, circuit voltage may remain near ground potential even when the ECM is not commanding the valve.
P0065 vs P0066 vs P0067
| Code | Description |
| P0065 | Air Assisted Injector Control Range/Performance |
| P0066 | Air Assisted Injector Control Circuit or Circuit Low |
| P0067 | Air Assisted Injector Control Circuit High |
Generally:
- P0065 indicates that system performance is outside the expected range.
- P0066 indicates a circuit malfunction or low-voltage condition.
- P0067 indicates a high-voltage condition, often associated with an open circuit.
Manufacturer-specific diagnostic strategies may vary.
How the ECM Detects P0066
The ECM can test the air-assist circuit during:
- Cold starts
- Engine warm-up
- Idle
- Low-speed operation
- Deceleration
- Specific emissions self-tests
The ECM compares commanded valve operation with actual circuit behavior.
P0066 may be stored when:
- Circuit voltage remains lower than expected
- Solenoid current exceeds the calibrated limit
- The control wire remains grounded
- The valve does not switch electrically
- The ECM commands the valve off but voltage remains low
- The solenoid coil has insufficient resistance
- The circuit intermittently shorts to ground
Depending on the vehicle, the Check Engine Light may illuminate after one failure or after the problem occurs during multiple drive cycles.
Symptoms of P0066
Possible symptoms include:
- Check Engine Light
- Rough cold start
- Unstable idle
- Hard starting
- Stalling after startup
- Hesitation
- Poor low-speed response
- Increased fuel consumption
- Rich or lean operation
- Strong fuel smell
- Increased exhaust emissions
- Failed emissions inspection
- Additional fuel-trim codes
Symptoms are often most noticeable:
- During cold weather
- Immediately after startup
- Before the engine reaches operating temperature
- At idle
- During light-throttle operation
Some vehicles may run normally despite storing P0066.
Common Causes of P0066
1. Control Wire Shorted to Ground
A damaged control wire may contact:
- The engine block
- Chassis
- Intake manifold
- Metal bracket
- Another grounded circuit
- Exposed shielding
This can keep circuit voltage low even when the ECM is not commanding the system.
2. Internally Shorted Air-Assist Solenoid
The solenoid coil may develop an internal short.
This lowers coil resistance and increases current draw.
Possible signs include:
- Blown fuse
- Hot solenoid
- Burned connector
- Very low resistance reading
- P0066 returning immediately
3. Failed Air-Assisted Injector Control Valve
The valve may bind mechanically and force the solenoid to draw more current than expected.
The valve may:
- Stick open
- Stick closed
- Move slowly
- Bind because of contamination
- Fail when hot or cold
4. Chafed or Melted Wiring
Inspect wiring near:
- Intake manifold
- Engine brackets
- Exhaust components
- Cylinder head
- Throttle body
- Harness retaining clips
Engine vibration can gradually wear through the insulation.
5. Corroded Connector
Corrosion may create unintended electrical paths or reduce terminal contact quality.
Inspect for:
- Green or white deposits
- Moisture
- Loose pins
- Backed-out terminals
- Burned plastic
- Damaged weather seals
6. Water Intrusion
Water inside the connector can temporarily short the control circuit.
This may produce an intermittent code that appears after:
- Heavy rain
- Engine washing
- Driving through standing water
- Cooling-system repairs
- Snow or road-salt exposure
7. Incorrect Replacement Solenoid
A replacement solenoid with incorrect resistance can draw more current than the ECM expects.
Universal components or incorrectly matched parts may cause P0066 even if they physically fit.
8. Incorrect Wiring Repair
Previous repairs may create:
- Crossed wires
- Poor crimps
- Unsealed splices
- Exposed conductors
- Incorrect connector pinning
- Insufficient wire size
A wire repair should restore both electrical integrity and environmental sealing.
9. Low System Voltage
Low battery or charging voltage may contribute to abnormal circuit readings.
Inspect:
- Battery state of charge
- Alternator output
- Battery terminals
- Engine grounds
- Chassis grounds
Low system voltage usually causes additional trouble codes as well.
10. Failed Fuse or Relay Circuit
Although a blown fuse is more commonly associated with a loss of power or high-circuit code, a partially damaged relay or overheated connection can create abnormal voltage behavior.
If multiple systems share the same power feed, additional codes may be stored.
11. ECM Control Driver Stuck Grounded
The ECM switches the solenoid ground through an internal transistor.
If this driver fails in the grounded position, circuit voltage may remain low continuously.
ECM failure is uncommon and should only be considered after the wiring, connector, valve, solenoid, power supply, and grounds have been tested.
Is P0066 Serious?
P0066 is generally considered a moderate-severity code.
The vehicle may continue operating, but the fault can affect:
- Cold-start performance
- Idle quality
- Fuel atomization
- Fuel consumption
- Exhaust emissions
- Catalyst operation
- Electrical circuit integrity
A shorted solenoid or control wire may also:
- Blow a fuse
- Overheat wiring
- Damage the connector
- Overload the ECM driver
A visible short circuit, burned connector, or repeatedly blown fuse should be repaired promptly.
Can You Drive With P0066?
You can usually drive with P0066 for a limited period if the engine runs normally.
However, avoid continued driving if the vehicle:
- Stalls repeatedly
- Runs extremely rich
- Misfires severely
- Produces a strong raw-fuel smell
- Blows the same fuse repeatedly
- Has burned or overheating wiring
- Shows reduced engine power
An unresolved rich-running condition may damage the catalytic converter over time.
A low-voltage control circuit may also become a larger electrical problem if damaged wiring continues rubbing against the engine.
How to Diagnose P0066
Step 1: Scan for Additional Codes
Check for related codes involving:
- P0065
- P0067
- Fuel trim
- Misfires
- MAP sensor
- MAF sensor
- Oxygen sensors
- Intake air leaks
- System voltage
- Injector circuits
Record freeze-frame data before clearing the codes.
Important freeze-frame values include:
- Engine coolant temperature
- Engine speed
- Intake manifold pressure
- Fuel trims
- Battery voltage
- Vehicle speed
Step 2: Confirm the Vehicle Uses Air-Assisted Injectors
P0066 is uncommon.
Before replacing anything, verify that the vehicle uses:
- An air-assist control valve
- Air-assisted fuel injectors
- An air-control solenoid
- Dedicated intake air passages
Consult manufacturer service information or a reliable wiring diagram.
Do not confuse this system with secondary air injection or a conventional fuel injector circuit.
Step 3: Locate the Air-Assist Valve or Solenoid
The component may be mounted near:
- Intake manifold
- Throttle body
- Fuel rail
- Injector harness
- Vacuum lines
- Air-cleaner housing
Trace the related hoses and wiring before disconnecting anything.
Photographing the hose routing first is preferable to discovering afterward that every hose suddenly looks equally plausible.
Step 4: Inspect the Wiring Harness
Look for:
- Bare copper
- Melted insulation
- Chafing
- Pinched wires
- Loose harness clips
- Oil contamination
- Previous repairs
- Contact with sharp brackets
Pay close attention to areas where the harness bends or touches the engine.
Step 5: Inspect the Connector
Disconnect the solenoid connector and inspect both halves.
Look for:
- Moisture
- Corrosion
- Loose terminals
- Bent pins
- Burned plastic
- Broken locking tabs
- Missing weather seals
Repair connector damage before condemning the valve.
Step 6: Check Battery and Charging Voltage
Measure battery voltage:
- With the engine off
- During cranking
- With the engine running
Low voltage during startup may affect system operation.
Also inspect:
- Battery terminals
- Engine ground straps
- Chassis grounds
- Alternator output
Step 7: Check the Fuse and Relay
Locate the fuse and relay supplying the air-assist circuit.
Verify:
- The fuse is intact
- The terminals are tight
- Power is present
- The relay switches correctly
- There is no heat damage
If the fuse is blown, do not repeatedly replace it without testing for excessive current or a short to ground.
Step 8: Measure Solenoid Resistance
Disconnect the air-assist solenoid.
Measure resistance across the solenoid terminals.
Possible results include:
- Near-zero resistance: internally shorted coil
- Resistance below specification: excessive current draw likely
- Infinite resistance: open solenoid
- Correct resistance: continue circuit testing
Always compare the reading with manufacturer specifications.
Step 9: Test the Control Wire for a Short to Ground
With the solenoid disconnected, test the ECM control wire for unintended continuity to ground.
If the wire remains grounded:
- Disconnect the ECM if the service procedure allows.
- Test the circuit again.
- Repair the harness if the short remains.
- Suspect the ECM driver only if the wiring tests correctly.
Do not probe module terminals carelessly. Incorrect testing can damage the ECM.
Step 10: Verify Power Supply
With the ignition on or the circuit commanded active, verify voltage at the solenoid power terminal.
If voltage is missing, inspect:
- Fuse
- Relay
- Power feed
- Junction connector
- Ignition circuit
P0066 often points toward the control side, but the complete circuit should still be checked.
Step 11: Perform a Voltage-Drop Test
A voltage-drop test can identify:
- Poor connections
- Corroded terminals
- Damaged splices
- Weak grounds
- Excessive resistance under load
A circuit may pass a basic continuity test and still fail while operating.
Step 12: Command the Solenoid With a Scan Tool
If bidirectional controls are available, command the air-assist solenoid on and off.
Monitor:
- Circuit voltage
- Current draw
- Commanded duty cycle
- Engine idle
- MAP reading
- Fuel trim
- Oxygen sensor response
If circuit voltage remains low when commanded off, suspect:
- A short to ground
- Internally shorted solenoid
- Stuck ECM driver
Step 13: Check Mechanical Valve Operation
The solenoid may click but still fail to move the valve correctly.
Inspect for:
- Carbon deposits
- Oil contamination
- Mechanical binding
- Broken internal parts
- Restricted air passages
A clicking solenoid proves that it can make a noise. It does not prove that it can regulate airflow.
Step 14: Inspect the Air and Vacuum Passages
Check for:
- Restricted hoses
- Collapsed lines
- Incorrect routing
- Carbon buildup
- Blocked manifold passages
- Disconnected vacuum source
Although P0066 is an electrical code, airflow restrictions may contribute to abnormal solenoid operation or related performance codes.
Step 15: Replace the Failed Component
Replace or repair the component confirmed by testing.
Possible repairs include:
- Air-assist solenoid replacement
- Air-control valve replacement
- Wiring repair
- Connector replacement
- Fuse or relay replacement
- Hose replacement
- Intake passage cleaning
Use OEM or high-quality direct-fit components.
Step 16: Verify the Repair
After completing repairs:
- Clear all diagnostic trouble codes.
- Allow the engine to cool completely.
- Perform a cold start.
- Monitor circuit voltage and solenoid current.
- Confirm stable idle operation.
- Check short- and long-term fuel trims.
- Complete the required drive cycle.
- Verify P0066 does not return.
Because the air-assist system often operates during cold starts, final verification may require the engine to remain off long enough to cool.
Common Repairs and Costs
| Repair | Estimated Cost |
| Wiring repair | $100–450 |
| Connector replacement | $75–275 |
| Fuse replacement | $10–50 |
| Relay replacement | $50–225 |
| Vacuum or air hose replacement | $50–200 |
| Air-assist solenoid replacement | $150–500 |
| Air-control valve replacement | $200–700 |
| Intake passage cleaning | $150–600 |
| ECM circuit diagnosis | $150–500 |
| ECM replacement | $1,000–2,500+ |
Costs vary depending on vehicle design, component accessibility, labor rates, and the extent of wiring damage.
Common Diagnostic Mistakes
Avoid these common mistakes:
- Confusing air-assisted injection with secondary air injection
- Replacing regular fuel injectors unnecessarily
- Replacing the solenoid without testing resistance
- Ignoring a control wire shorted to ground
- Replacing a blown fuse without finding the cause
- Assuming a clicking solenoid works correctly
- Missing corrosion inside the connector
- Ignoring low charging voltage
- Skipping manufacturer wiring diagrams
- Replacing the ECM before testing the circuit
- Clearing freeze-frame data before recording it
The code contains the word “injector,” but that does not mean the correct first step is purchasing a full set of fuel injectors.
Vehicles Commonly Affected
P0066 is relatively uncommon because air-assisted injector systems are not used on most modern vehicles.
It may appear on certain:
- Subaru models
- Toyota models
- Lexus models
- Mitsubishi vehicles
- European vehicles
- Low-emission engine packages
- Older specialized fuel-injection systems
Exact applications vary by:
- Model year
- Engine
- Emissions certification
- Market
- Fuel-injection design
Always verify that the vehicle uses an air-assisted injector system before ordering parts.
How to Prevent P0066
Reduce the likelihood of P0066 by:
- Keeping engine wiring secured
- Repairing oil or coolant leaks
- Protecting connectors from moisture
- Using OEM-quality replacement components
- Avoiding unsealed wiring splices
- Inspecting harnesses during intake repairs
- Maintaining battery and alternator health
- Replacing brittle vacuum hoses
- Cleaning intake passages when required
- Investigating repeated fuse failures
Frequently Asked Questions
What does P0066 mean?
P0066 means the ECM has detected a malfunction or low-voltage condition in the air-assisted injector control circuit.
What is an air-assisted injector?
An air-assisted injector system introduces controlled air near the fuel injector or intake port to improve fuel atomization and cold-start combustion.
Is P0066 serious?
P0066 is moderately serious.
The vehicle may remain drivable, but a shorted circuit can damage wiring, blow a fuse, or overload the ECM driver.
Can I drive with P0066?
Usually, yes, if the engine runs normally.
Avoid driving if the engine stalls, runs extremely rich, misfires severely, or has burned wiring.
What commonly causes P0066?
Common causes include:
- Control wire shorted to ground
- Internally shorted solenoid
- Failed air-assist valve
- Damaged wiring
- Corroded connector
- Incorrect replacement part
- ECM driver failure
Will replacing the fuel injectors fix P0066?
Usually not.
P0066 generally refers to the air-assist valve or solenoid circuit rather than the conventional fuel injectors.
Can a shorted solenoid cause P0066?
Yes.
An internally shorted solenoid can draw excessive current and pull circuit voltage below the ECM’s expected range.
Can P0066 blow a fuse?
Yes.
A grounded control wire or shorted solenoid may draw enough current to blow a shared fuse.
Can low battery voltage cause P0066?
It can contribute to abnormal circuit behavior, although a wiring or solenoid fault is usually more likely.
What is the difference between P0065 and P0066?
P0065 indicates an air-assisted injector system range or performance problem.
P0066 indicates an electrical circuit malfunction or low-voltage condition.
What is the difference between P0066 and P0067?
P0066 normally indicates low circuit voltage or excessive current.
P0067 indicates high circuit voltage or insufficient current, often caused by an open circuit.
Can P0066 cause rough idle?
Yes.
Improper injector air assistance can produce rough or unstable idle, especially during cold starts.
Can P0066 cause an emissions failure?
Yes.
The Check Engine Light and incomplete readiness monitors may prevent the vehicle from passing an emissions inspection.
Final Thoughts
The P0066 code means the ECM has detected an electrical malfunction or low-voltage condition in the air-assisted injector control circuit.
The most common causes include a control wire shorted to ground, an internally shorted solenoid, a damaged connector, chafed wiring, or a failed air-assist valve.
Begin diagnosis by confirming that the vehicle actually uses an air-assisted injector system. Then inspect the wiring and connector, measure solenoid resistance, test for a short to ground, verify power, and command the circuit with a capable scan tool.
Do not automatically replace the fuel injectors simply because the code includes the word “injector.” P0066 generally concerns the airflow control system surrounding the injectors—not the injectors themselves. The parts cannon may be entertaining, but a multimeter is usually cheaper.
Related Articles
Browse the complete Pro Street diagnostic libraries:
- Complete OBD-II Guide: https://my.prostreetonline.com/dtc-obdii/
- B-Code Guide: https://my.prostreetonline.com/dtc-doctor/b-code-obd-ii-codes/
- C-Code Guide: https://my.prostreetonline.com/dtc-doctor/c-code-obd-ii-codes/
