P0066
Air Assisted Injector Control Circuit or Circuit LowP0066 is a generic OBD-II powertrain diagnostic trouble code: Air Assisted Injector Control Circuit or Circuit Low. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0066 means
Code P0066 is set when the powertrain control module (PCM) detects a lower-than-expected voltage in the air assisted injector control circuit. The air assisted injection system uses a solenoid-controlled valve to route fresh air to the fuel injectors during cold-start warm-up, improving fuel atomisation and catalyst light-off. The PCM supplies a control voltage to the solenoid and monitors the circuit feedback. A low-voltage condition — typically a voltage reading below the calibrated minimum threshold when the PCM commands the circuit active — suggests a short to ground in the wiring, an open in the supply side, excessive resistance in connectors, or an internal short in the solenoid coil itself. Unlike P0065 which is a performance fault, P0066 is a hard circuit-level fault: the electrical signal the PCM monitors is measurably outside the expected voltage window at the low end. The system is primarily active during cold starts at coolant temperatures below approximately 160°F, so the fault may be intermittent and only present on cold-start conditions. Extended operation with a failed air assist circuit can result in slightly richer cold-start emissions and increased catalyst warm-up time, but does not typically cause immediate mechanical damage.
Common causes
Most-frequently reported root causes when P0066 is logged.
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1
Short to ground in the solenoid control wire between the PCM and the air assist valve connector
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2
Open circuit in the voltage supply wire to the solenoid (broken wire, blown fuse, or failed relay)
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3
Corroded or damaged solenoid connector causing high resistance on the supply side
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4
Internal short-to-ground within the air assisted injector solenoid coil winding
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5
Chafed wiring harness grounding against a chassis or engine ground point
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6
PCM output driver failure causing incorrect low-side switching behavior
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7
Water or moisture ingress into the solenoid connector causing a leakage current path to ground
Symptoms drivers notice
How to diagnose P0066
A typical diagnostic flow when this code is present.
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1
Visually inspect the air assisted injector solenoid wiring harness for chafing, melted insulation, or corroded connector terminals
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2
Measure the solenoid resistance at the connector; typical spec is 20–40 Ω — near-zero resistance indicates an internal coil short to ground
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3
With the ignition on and the PCM commanding the circuit, measure voltage at the solenoid connector; a reading significantly below battery voltage on the supply wire points to an open or high-resistance supply path
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4
Perform a voltage drop test across the supply and return wires to isolate resistance in connectors or splices
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5
Check for a blown fuse in the circuit supplying the air assist solenoid
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6
Repair any wiring faults found; clear the code and confirm it does not return on a cold start
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7
If wiring and solenoid are both within spec, suspect PCM output driver failure and consult manufacturer-specific diagnostic procedures
Related powertrain codes
Frequently asked questions
What is the difference between P0066 and P0065?
P0065 is a performance/range code — the circuit is electrically intact but the system is not producing the expected functional result (no MAP change when valve opens). P0066 is a hard circuit fault — the PCM measures a voltage that is too low, indicating a wiring or component electrical failure.
Can a bad solenoid cause P0066?
Yes. An internal short-to-ground in the solenoid coil will pull the circuit voltage low below the PCM's threshold and set P0066. Testing solenoid resistance with a multimeter is one of the first checks to perform.
Is it safe to drive with P0066?
The vehicle is generally safe to drive as the air assist system only operates briefly during cold starts. However, prolonged cold-start lean or rich conditions and increased catalyst warm-up time can accelerate wear on the catalytic converter if ignored.
Could moisture in the connector cause this code?
Yes. Water or condensation in the solenoid connector can create a leakage current path to ground, pulling the circuit voltage below the PCM's low threshold and triggering P0066. Thoroughly cleaning, drying, and applying dielectric grease to the connector can resolve moisture-related faults.
Disabling P0066 in software
RaceTune can permanently disable P0066 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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