P21D8

Code

Generic P — Powertrain

Cylinder 10 Injector B Circuit/Open

Brand: Generic

Type: P — Powertrain

Views: UK: 24 EN: 29 RU: 26

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Page language: EN

Causes

Open circuit in injector wiring (broken wire, connector failure)
Intermittent or poor connector contact (corrosion, bent terminal, water ingress)
Short to ground or short to battery/power in the injector circuit
Failed fuel injector (solenoid open or internally shorted)
Blown fuse or faulty injector relay/power supply
Faulty or damaged engine control module (ECM) driver for the injector

Symptoms

Check Engine Light (CEL) illuminated with code P21D8
Rough idle or vibration caused by misfire on Cylinder 10
Loss of power or hesitation under load
Increased fuel consumption and elevated hydrocarbon emissions
Single-cylinder misfire stored in freeze frame or misfire counters

What to check

Read freeze frame and manufacturer-specific data with a scan tool; note misfire counters and live injector commands
Visually inspect injector connector, wiring harness, and nearby components for damage or corrosion
Check fuses and injector power feed/relay for proper operation
Measure injector coil resistance with an ohmmeter at the connector and compare to specification
Back-probe injector connector while cranking/running to verify commanded pulse (noid light or oscilloscope)
Perform continuity check between injector connector and ECM pin to locate opens/shorts

Signal parameters

Injector coil resistance (typical ranges): low-impedance: ~0.5–3 Ω; high-impedance: ~10–16 Ω — consult vehicle spec
Command waveform: pulsed square wave from ECM (0–12 V or switching to ground)
Pulse width/dwell: commonly 1–10 ms depending on load and RPM
Duty cycle: 0% at idle (when not injecting) up to ~80% at high load — varies by engine
Supply voltage at injector power pin: battery voltage (~12 V) with key on and during cranking

Diagnostic algorithm

Connect a scan tool and confirm P21D8, note freeze frame and misfire counts for Cylinder 10
Visually inspect the injector connector, pins and wiring for corrosion, loose terminals, heat damage or fluid intrusion
Check relevant fuses and injector power/relay circuits for proper voltage with key on
Measure injector coil resistance at the connector; compare to OEM spec. If resistance is open/infinite, suspect an internal injector fault
Back-probe the injector connector while cranking/running: verify the ECM is commanding the injector with a pulse (use noid light or oscilloscope). If there is no pulse but ECM command is present in scan tool, check wiring to ECM
Perform continuity test between injector connector and ECM driver pin. Repair any open or high-resistance connections found
Inspect for short to power or ground by measuring resistance between injector control wire and battery positive/ground with harness disconnected
Perform a wiggle test of the harness while observing live data or a noid light to detect intermittent faults
Swap the suspect injector with a known-good injector from another cylinder (if practical). If the fault follows the injector, replace the injector. If it remains on Cylinder 10, continue wiring/ECM diagnosis
Repair: repair or replace damaged wiring, connectors, or fuse/relay. Replace injector if internally open/shorted. After repairs, clear codes and road test to verify repair
If wiring and injector test good but circuit still faults, consider ECM driver fault; confirm wiring/grounds first, then consult manufacturer procedures for ECM testing or replacement

Likely causes

Connector corrosion or loose pin at Cylinder 10 injector
Broken or chafed wire between injector connector and ECM
Failed injector coil (open winding)
Poor ground or missing injector supply voltage

Fault status

⚠️ Status

P21D8 — Cylinder 10 Injector B Circuit/Open. The ECM detected an open or abnormal condition in the injector control circuit for cylinder 10; electrical fault between ECM and injector or injector internal failure possible.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours