P0261 — Cylinder 1 Injector Circuit Low (7.3 Power Stroke)

P0261 on a 7.3L Power Stroke (1999–2003 Super Duty/Excursion) means the PCM/IDM has detected low electrical current or voltage in the cylinder 1 injector…

Updated September 2, 2026

What it means

The engine control module has detected low electrical current or voltage in the injector circuit for cylinder 1. On this platform the injectors are HEUI, IDM-driven, with UVCH known as a failure point. Typical on 7.3L Power Stroke (1999-2003 Super Duty/Excursion) — HEUI, IDM-driven injectors, UVCH known failure point.¹

Most likely causes

1. Open or high-resistance in cylinder 1 injector circuit (UVCH connector/pigtail, injector solenoid terminals, or IDM output wiring). — The 7.3 uses HEUI injectors under the valve covers fed by UVCH; oil-soaked/chafed UVCH is a known failure point for injector and glow-plug circuit codes. Per-cylinder circuit faults are P0261/P0262 for cyl 1, etc., with IDM/output-circuit faults P1660/P1662/P1663.¹²
2. Faulty cylinder 1 injector solenoid/electrical coil (low current draw). — PCM/IDM monitors injector circuit current; low current indicates an open or weak solenoid. This is a per-cylinder injector circuit fault on HEUI, IDM-driven injectors.¹²
3. Injector Driver Module (IDM) output fault for cylinder 1 driver. — Injectors are driven by the IDM (driver-side fender). IDM/output-circuit faults are P1660, P1662, P1663. A failing IDM can present as a single-cylinder low-circuit code.¹
4. Corrosion/moisture in under-valve-cover harness or valve-cover pass-through connector for cylinder 1. — UVCH connectors and pigtails live under each valve cover and are known to fail from oil exposure and chafing, commonly setting injector circuit codes.¹
5. Poor PCM/IDM power/ground or wiring harness chafing affecting cylinder 1 injector control wire. — Electrical integrity of the IDM-to-UVCH path is required for proper injector current monitoring; intermittent opens cause low-circuit detection.¹

How to diagnose

1. Verify code and freeze-frame; confirm P0261 is present and not accompanied by ICP/IPR or IDM codes.
2. Inspect IDM power/ground and main engine harness for chafing/corrosion; check IDM connector pins for oil/moisture.
3. Perform injector circuit electrical tests at the IDM connector and UVCH pass-through for cylinder 1 (continuity, resistance, isolation to ground). Compare to other cylinders.
4. Inspect under-valve-cover harness and injector solenoid terminals for oil saturation, burnt pins, or damaged pigtails; look for signs of arcing.
5. Swap IDM output test with a known-good IDM if available, or monitor IDM driver command vs. actual current with a scan tool capable of injector buzz test.
6. If circuit tests normal, evaluate cylinder 1 injector solenoid resistance and compare to spec and other cylinders.
7. Clear codes, perform injector buzz test and cylinder contribution test to confirm fault follows cylinder 1.

Parts and tools you may need

• Digital multimeter
• Scan tool with Ford/HEUI injector buzz and cylinder contribution tests
• Injector breakout box / IDM harness test adapter
• Torx and socket set for valve cover removal
• Dielectric grease
• Replacement UVCH and injector pigtail kit
• Replacement IDM (if testing indicates driver fault)
• Replacement HEUI injector (if solenoid electrical fault confirmed)

Rough cost and difficulty

Parts and labor vary by region and shop. UVCH/pigtail service is common and less costly than injector replacement; IDM replacement is more expensive. Diagnosis should confirm the electrical fault before replacing the injector.

When to see a pro

If the injector buzz test shows no response, or if multiple injector codes appear, have a qualified diesel technician evaluate the IDM, UVCH, and high-pressure oil system. Do not attempt high-pressure oil or fuel line service without proper training and safety equipment.

Sources