Comprehensive diagnostic guide for P0261 Cylinder 1 Injector Circuit Low

What This Code Means

• P0261 is an OBD-II powertrain trouble code indicating a fault in the injector circuit for cylinder 1, specifically a low signal to the injector drive circuit. In the open-source code definitions, it is described as (injector control circuit - cylinder 1 - low signal). In other sources, it's described as Cylinder 1 Injector Circuit Low, i.e., the injector drive signal for cylinder 1 is not being driven high as commanded. These descriptions reflect the same underlying issue: the PCM/ECU is not delivering the expected injector signal to cylinder 1. Source references: OBD-II Diagnostic Trouble Codes, Powertrain Codes, and the Open Source OBD2 code definition for P0261.
  • Wikipedia: OBD-II - Diagnostic Trouble Codes and Powertrain Codes sections describe how DTCs classify faults in engine management systems, including injector circuits.
  • Open Source code definition: P0261 - (Injector control circuit - cylinder 1 - signal low).

Symptoms

• MIL (Check Engine Light) is on.
• Cylinder 1 misfire symptoms: rough idle, engine stumble, hesitation, or reduced power under load.
• Lost or reduced engine performance in the affected cylinder; potential minor impact on fuel economy.
• In some cases, you may see P0301 (cylinder 1 misfire) in conjunction with P0261 or as a related observation when the injector signal remains low and the cylinder isn't receiving expected fuel.
  Note: Symptoms can vary by vehicle and may occur with other injector-related codes or misfire codes. The diagnostic approach is to confirm the injector signal behavior and then isolate whether the issue is mechanical, electrical, or PCM/driver related.

Probable Causes

These percentages are approximate and vehicle-specific; they reflect common failure patterns observed in the shop and generalized industry experience, not a universal statistic.

• Faulty injector (cylinder 1) or injector electrical fault (coil, internal short): ~40%
• Wiring harness/connector issue to cylinder 1 injector (damaged insulation, corrosion, loose pin, broken conductor): ~25%
• PCM/ECU injector driver or output channel fault (ECU stuck low or unable to source/sink proper current): ~15%
• Power supply issues to the injector circuit (fusing, relay operation, poor grounding, battery/VR electrical problems): ~10%
• Mechanical issues in cylinder 1 (low compression or excessive leakage) indirectly causing the injector circuit to appear low (more typical with broad misfire codes; less common for P0261 alone): ~5%
• Other/uncategorized or intermittent faults (noise, wiring intermittents, etc.): ~5%

Diagnostic Approach

1) Confirm the fault

• Read DTCs with a professional scan tool. Note any accompanying codes (e.g., P0301, P0300, misfire-related codes, or other injector codes).
• Review freeze-frame data to understand operating conditions at the time of the fault (engine speed, load, fuel trim, injector pulse width, etc.).

2) Gather symptom context

• Does the customer describe rough idle, misfire at idle or under load, or hesitation?
• Is there a fault only when engine is cold or under certain RPM, or is it constant?

3) Visual and basic electrical inspection

• Inspect cylinder 1 injector connector and wiring harness for damaged insulation, corrosion, pin damage, or loose connections.
• Inspect related fuses/relays for injector power or ground circuits as defined by vehicle service information.
• Inspect injector mounting and the injector's O-ring seals, as a leaking injector can alter circuit readings indirectly.

4) Compare injector behavior (is the fault injector-specific?)

• Swap or swap-test injector 1 with another known-good injector in the same bank (or swap the cylinder 1 injector with cylinder X) and re-check the code and symptoms.
  • If the fault follows the injector, the injector is suspect.
  • If the fault stays on cylinder 1 despite swapping injectors, this points to wiring, connector, or ECU/driver issues.
• If feasible, measure voltage when the injector is commanded to open (via scan tool + oscilloscope). Look for the expected signal amplitude and duty cycle corresponding to engine speed and load.
• If the injector is a ground-switched type, verify the injector's ground path; if it's voltage-supplied, verify the supply to the injector. Look for voltage drop and grounding issues.

5) Electrical test of injector circuit

• Measure the resistance of cylinder 1 injector with the injector connector unplugged (reference from fuel injector testing practice). Compare against the manufacturer's spec. A resistance out of spec can indicate a faulty injector.
• With injector connector plugged in, measure the injector drive signal from the PCM (or use a current clamp while the injector is commanded). If the current draw is significantly lower than expected, that could indicate the injector is not energizing properly or a driver issue.
• Check the signal waveform using an oscilloscope or a high-quality scope-compatible test pedal. A weak or non-existent drive pulse when commanded indicates a driver or wiring problem; a proper pulse width but reduced current can indicate a wiring resistance issue or a partially damaged injector.

Safety Considerations

• Work in a well-ventilated area; keep ignition sources away from fuel vapors.
• Depressurize the fuel system per the vehicle's service procedure before disconnecting fuel lines or injectors.
• When probing electrical circuits, disconnect the battery as appropriate to avoid short circuits or damaging the ECU.
• Follow OEM service procedures for injector replacement and fuel system work; incorrect reassembly can cause leaks or future faults.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: Internal combustion engine, OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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