P21E3

Code

Generic P — Powertrain

Cylinder 4 Injector B Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 23 RU: 17

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Page language: EN

Causes

Short in the injector control wiring to battery positive (B+)
Corroded, damaged or pushed-out connector at cylinder 4 injector
Internal short/failure of the cylinder 4 injector (coil or internal electronics)
Faulty ECM/injector driver (control transistor stuck open to B+ or damaged)
Poor or missing ground in the injector supply or engine ground circuit
Aftermarket harness or recent repairs with incorrect connections

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Engine misfire or rough running localized to cylinder 4
Reduced engine power or hesitation under load
Poor fuel economy
Hard starting or stumble during cranking (in some cases)

What to check

Read and record all stored/active freeze-frame data and related codes.
Visual inspection of wiring and connector at cylinder 4 injector for damage, corrosion, or pin push-out.
Backprobe injector connector: check constant supply (battery) voltage on the supply pin with key ON.
Backprobe/control pin check with oscilloscope or DVOM while cranking/idling to verify pulse waveform and voltage level.
Measure injector coil resistance (with connector disconnected) and compare to spec.
Perform wiggle test of harness while monitoring live data or MIL for changes.

Signal parameters

Injector supply pin: ~battery voltage (~12 V) with key ON
Injector control/driver pin: should be a pulsed waveform (0 V to near battery) controlled by ECM during injection
Typical injector coil resistance: vehicle-specific (commonly ~1–16 ohms for low-impedance, higher for high-impedance units) — check factory spec
Pulse width: tens to hundreds of milliseconds depending on load/engine speed
No DC high voltage (steady B+) should be present on the control/driver pin when the ECM is commanding low or pulsed; steady high indicates fault

Diagnostic algorithm

Retrieve P21E3 and any related codes; note freeze-frame data and engine conditions when fault set.
Visual inspection: check harness routing, chafing, heat damage, and connector condition at cylinder 4 injector and nearby chassis/engine-ground points.
With ignition ON (engine OFF) backprobe the injector connector: verify the supply pin has battery voltage and the control pin is not stuck at battery voltage.
Start engine or crank and observe the control pin with an oscilloscope (preferred) or scanner with injector status: look for proper pulsed waveform. A steady high indicates circuit high.
With connector disconnected, measure injector coil resistance and compare to specification. An abnormally low resistance suggests an internal short.
Disconnect the suspect injector and clear codes. If the code does not return after a drive cycle and symptoms change, the injector/harness is likely the cause.
Perform continuity and short-to-B+ tests on the control wire back to the ECM: check for low resistance to battery positive (indicates short).
Swap the cylinder 4 injector with another known-good injector (same type) and repeat testing to see if the code follows the injector or stays on cylinder 4 circuit.
If wiring and injector check OK and the fault follows the ECM driver pin, consider ECM driver failure — verify with manufacturer guidance before ECM replacement.
Repair or replace damaged wiring, connectors, or injector as required. After repair, clear codes and verify repair with a road test and re-scan.

Likely causes

Wiring short to battery (pinched or chafed harness near engine)
Corroded/loose injector connector at cylinder 4
Defective injector with internal short
ECM driver transistor failure (less likely than wiring/injector)
Intermittent ground or poor connector mating

Fault status

⚠️ Status

P21E3 — Cylinder 4 Injector B Circuit High: ECM detected an over-voltage condition on the Cylinder 4 injector control circuit. MIL set. Investigation of wiring, connector, injector and ECM driver required.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2 hours