P0066 – Air Assisted Injector Control Circuit or Circuit Low

P0066 is an OBD-II trouble code that points to a problem with the intake air-assisted injector control circuit, often called the “air assist injector” or “intake air control for injectors.” When this circuit doesn’t behave the way the ECU expects, you can get rough running, hard starting, and poor fuel economy. In this guide, you’ll learn exactly what P0066 means, the most common causes, what symptoms to watch for, how a mechanic diagnoses it, and what repairs usually cost so you can decide your next step with confidence.

What Does P0066 Mean?

P0066 stands for “Air Assisted Injector Control Circuit Low.” Your engine control unit (ECU) monitors the electrical signal going to the air-assisted injector or the air control solenoid that helps atomize fuel at lower engine speeds and cold starts. When the ECU sees voltage or feedback lower than the expected range, it stores P0066 and usually turns on the check engine light.

This code is most common on vehicles that use an air-assist system to improve fuel atomization and emissions. The problem can be as simple as a bad connector or as involved as a failed solenoid or wiring harness fault.

Quick Reference

• Code: P0066
• Definition: Air Assisted Injector Control Circuit Low
• Typical Severity: Low to moderate (drivable but not ideal)
• Common Causes: Faulty air-assist injector/solenoid, wiring issues, poor connections
• Common Symptoms: Rough idle, hard cold start, higher fuel consumption

Real-World Example / Field Notes

In the shop, I usually see P0066 on smaller gasoline engines where owners complain that the car is hard to start in the morning and idles rough for the first minute. One Subaru owner came in convinced they needed a new fuel pump. Fuel pressure checked out fine, but scan data showed the ECU repeatedly commanding the air-assist injector with no change in current. A quick voltage drop test at the connector revealed high resistance in the ground wire from corrosion under the battery tray. Cleaning the connection and repairing the wire cleared P0066 and restored smooth cold starts.

Symptoms of P0066

• Check engine light on – P0066 will usually trigger the MIL and store a permanent code.
• Rough idle – Especially noticeable during cold start or right after starting the engine.
• Hard cold starting – Longer cranking time or needing to press the gas pedal to keep it running.
• Poor fuel economy – Incomplete atomization can cause richer mixtures and more fuel use.
• Hesitation on acceleration – Light throttle tip-in can feel sluggish or stumble slightly.
• Increased tailpipe emissions – May show up as a failed emissions or smog test.
• Occasional engine misfire – Not always severe, but you may feel light shaking at idle.

Common Causes of P0066

Most Common Causes

• Faulty air-assisted injector or solenoid – The internal coil can fail, causing low current or an open circuit.
• Damaged wiring to the injector – Broken, chafed, or shorted wires between the ECU and the air-assist injector circuit.
• Corroded or loose connector – Moisture intrusion at the injector or solenoid plug is very common.
• Poor ground connection – High resistance in ground circuits can cause low voltage at the component.
• Previous repairs or modifications – Aftermarket intake or engine work sometimes pinches or stretches the harness.

Less Common Causes

• ECU internal fault – The driver transistor for the air-assist circuit can fail, though this is rare.
• Blown fuse or relay issue – If the air-assist injector shares power with other components, a blown fuse can drop voltage.
• Intake air leaks – While not a direct electrical cause, vacuum leaks can confuse diagnostics and create similar symptoms.
• Water intrusion in harness – Vehicles driven in heavy rain, snow, or off-road conditions can get water inside loom sections.

Diagnosis: Step-by-Step Guide

To properly diagnose P0066, you’ll want a basic tool set: a quality OBD-II scan tool (preferably with live data and Mode $06), a digital multimeter, wiring diagrams for your vehicle, and, ideally, a test light. Contact cleaner and dielectric grease are also handy for cleaning connectors.

1. Confirm the code and freeze-frame data. Connect your scan tool, read P0066, and note engine temperature, RPM, and vehicle speed when the code set. This tells you if it occurs mainly on cold starts or under certain conditions.
2. Check for related codes. Look for other injector, air-fuel ratio, or sensor codes (especially P0065/P0067). Multiple codes can point to a shared power or ground issue.
3. Perform a visual inspection. Locate the air-assisted injector or air-assist solenoid (service manual or repair database will help). Inspect the connector, wiring harness, and nearby components for damage, rubbing, or oil contamination.
4. Inspect and clean connectors. Unplug the connector and check for corrosion, bent pins, or moisture. Clean with electrical contact cleaner, let it dry, and apply a small amount of dielectric grease before reconnecting.
5. Check power and ground at the component. With the key on (engine off), use a multimeter to verify proper battery voltage on the power wire and a solid ground on the ground wire. Compare readings to specs in your service information.
6. Measure resistance of the air-assist injector/solenoid. With the connector unplugged, measure coil resistance across the terminals. Compare to factory specs. An open circuit, short, or significantly out-of-range value indicates a bad component.
7. Perform a wiggle test on the harness. While monitoring voltage or continuity, gently move the harness and connector. If readings jump or the engine stumbles, you likely have an intermittent wiring fault.
8. Check ECU control side. Using a wiring diagram, verify continuity from the injector connector back to the ECU pin. If continuity is good and the component tests fine, the issue may be in the ECU driver.
9. Use scan data or Mode $06 if available. Some vehicles show commanded vs. actual current or test results for the injector circuit. Look for failed or marginal test values that confirm a weak circuit.
10. Clear codes and perform a road test. After repairs or tests, clear P0066 and drive under similar conditions to see if the code returns.

Pro tip: Always load-test circuits, not just check for voltage. A circuit can show 12 V with no load but drop way down when the injector is commanded on. Using a test light or monitoring voltage while the ECU activates the circuit will catch weak connections that a simple static reading misses.

Possible Fixes & Repair Costs

Most repairs for P0066 are straightforward once you pinpoint the fault. Common fixes include replacing the air-assisted injector or solenoid, repairing or replacing damaged wiring, cleaning and tightening connectors, or in rare cases replacing the ECU. A new air-assist injector or solenoid typically runs $100–$300 parts and labor. Wiring repairs can range from $80–$250 depending on access and severity. ECU replacement and programming is the most expensive, often $600–$1,200 or more. Final cost depends on vehicle make, part availability, labor rates in your area, and whether additional issues (like vacuum leaks) are found during diagnosis.

Can I Still Drive With P0066?

In most cases, you can still drive with P0066, but the engine may not run as smoothly as it should, especially on cold starts. You might notice rough idle, hesitation, and lower fuel economy. While it’s not usually an immediate “stop now” situation, you shouldn’t ignore it for long. Running with poor atomization and richer mixtures can foul spark plugs, damage the catalytic converter over time, and make the car fail an emissions test. If the engine runs very rough or stalls, limit driving and get it checked as soon as possible.

What Happens If You Ignore P0066?

If you ignore P0066, the engine may continue to run rich during certain conditions, leading to carbon buildup, fouled plugs, and potential long-term damage to the catalytic converter. Over time, you can also see worsening drivability issues, more frequent cold-start problems, and additional fault codes related to fuel trim or misfires.

Last updated: February 13, 2026

Vehicles Commonly Affected by P0066

P0066 shows up most often on gasoline engines that use an air-assist system for fuel injectors. You’ll commonly see it on Subaru models (Impreza, Forester, Outback), some Mitsubishi and Mazda vehicles, and a few Toyota and Honda engines that use similar intake air-assisted injection strategies. Smaller four-cylinder engines and compact crossovers are frequent visitors in the shop with this code, especially in regions with heavy corrosion or where wiring harnesses are exposed to road salt and moisture.