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Air Assisted Injector Control Circuit or Circuit Low

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What P0066 means

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The control unit reports “Air Assisted Injector Control Circuit or Circuit Low”. Code P0066 identifies the area to investigate, but it does not prove that one named part has failed without a confirming test and vehicle context.

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General information, not a diagnosis or repair instruction. Always confirm with a qualified mechanic for your specific vehicle.

When a warning involves the fuel injector circuit, the order of checks matters more than the parts list.

What this code does not prove

This code describes a system observation about the fuel injector circuit, not a verdict that one named part has failed. The same code can have more than one cause, so the paths must be separated by testing.

Where the diagnosis usually goes next

Check the signal at its source

Code P0066 describes a condition in the fuel injector circuit: the signal reads at or near the bottom of its expected electrical range. That names where to look, but it does not by itself confirm which part has failed.

How to tell the paths apart: Read the live data and compare it against the expected range for current conditions before replacing anything.

Rule out wiring and the connector

Codes of this shape often come from damaged wiring, a corroded connector, or a poor ground on the fuel injector circuit — not the component itself.

How to tell the paths apart: Inspect the wiring and connector for the fuel injector circuit for damage and a secure ground before replacing anything.

Before you drive on

A code about the fuel injector circuit alone cannot give a universal verdict for this specific vehicle. If the car is running normally, limit use to what is needed for diagnosis; any change in behaviour changes the decision.

Stop and arrange a check if

  • the engine is misfiring, losing power, or stalling
  • smoke, a fuel/burning smell, or overheating appears
  • the fuel injector circuit's behaviour is clearly getting worse rather than staying stable

Before authorising a repair

  • Bring the complete code list and freeze-frame data relevant to the fuel injector circuit
  • Ask for the test that confirms a fault in the fuel injector circuit — not just the code alone

What to check first

  1. Read live data for the fuel injector circuit and compare it against the expected range for current engine conditions.
  2. Inspect the wiring, connector, and ground for the fuel injector circuit before replacing the component.
  3. A fuel injector circuit should show a consistent, repeatable resistance and a clean on/off switching pattern (a square-wave signal) with no shorts to power or ground. A circuit with abnormal resistance, a signal that never fully switches, or one cylinder behaving differently from its neighbors under the same commanded pulse width points to that injector, its driver circuit, or its wiring rather than a fuel-quality or mechanical issue — comparing pulse width and resistance across cylinders isolates the odd one out.
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Frequently asked questions

Does this code tell me the fuel injector circuit has failed?

No. It points to the fuel injector circuit; the cause still needs to be confirmed by a measurement or targeted test.

What should I bring to the workshop?

The complete code list, freeze-frame data, and the conditions around the fuel injector circuit — that gives the technician context, not a guess.

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