P0201 — Injector Circuit/Open Cylinder 1 (OBD Diesel)
P0201 means the cylinder 1 injector circuit has an open or malfunction detected by the OBD driver monitoring system. Treat this as a circuit integrity diagnosis first — harness opens and oil-intruded connectors are more common than injector failure on diesel pickup platforms.
Fault Overview
P0201 is set when the ECM detects an open or malfunction in the cylinder 1 injector control circuit. On OBD diesel pickups, this causes a cylinder misfire, rough idle, and power loss. Extended misfire can damage the DPF with unburned fuel. Harness flex opens, after-service connector seating issues, and oil-contaminated valve cover connectors are the most common causes.
Engine & Platform Coverage
Symptoms
- •Cylinder 1 misfire active
- •Rough idle on diesel pickup
- •P0201 injector circuit fault
- •Power loss
- •DPF soot loading increase from misfire
Possible Causes
- 1.Open wire — broken wire at harness flex point
- 2.Failed injector — internal coil or piezo open circuit
- 3.Connector not fully seated — after-service connector seating issue
- 4.Oil intrusion in valve cover connector — corrosion causing open
- 5.ECM driver failure — rare, last resort
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Single Action Gate — Visual Connector Inspection: Inspect injector connector for oil intrusion, corrosion, or incomplete seating. After-service connector seating issues are a common cause of P0201 on diesel pickups.
- 2Binary Decision Gate — Injector Resistance Check: Measure injector coil resistance versus OEM specification. An open reading (infinite resistance) confirms internal injector failure. Within spec means the injector is electrically good.
- 3Override Manager — Harness Continuity Test: With injector disconnected, measure continuity from injector connector to ECM connector. Any open in the harness confirms wiring fault before injector replacement.
- 4Safety Interceptor — Short Checks: Verify no short to ground or power on the injector circuit. Shorts can damage ECM drivers.
- 5Resume Point — Controlled Swap Test: If OEM procedure allows, swap injector to another cylinder and verify fault follows the injector. Only perform if safe and documented per OEM procedure.
- 6Verification Loop — Post-Repair Circuit Confirmation: After repair, verify injector circuit resistance and continuity are within spec before starting engine.
- 7Repair Ledger Output — Injector Replacement: Replace injector only after circuit is proven good. Note: some platforms require injector coding after replacement.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Clean and reseat connector if oil intrusion or incomplete seating is found.
- 2Repair any harness opens or broken wires found.
- 3Replace injector if internal coil failure is confirmed.
- 4Program injector code if required by OEM after replacement.
- 5Seal valve cover connector against future oil intrusion.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
Treat P0201 as a circuit integrity diagnosis first. Inspect connectors, measure resistance, and verify harness continuity before replacing the injector. The AI Diesel Tech FSM enforces electrical open proof before the injector replacement branch opens.