P21EF: Cylinder 10 Injector B Circuit High

P21EF stands for a high reading on the B injector circuit of cylinder 10. No public manufacturer document covers it, so the exact threshold is unknown; the closest documented model is GM's LT5 V8, where a switched-off low side within 0.5 Ohm of supply counts as a short and sets a Type A fault. A vehicle needs ten or more cylinders and two injectors each to use the code, which so far is documented only for Lamborghini's V10 in the Huracán. A short to power keeps the injector from ever opening, and switching against it strains the output stage, which may need replacement if the fault persisted. When P21EF shows up together with other high codes on the same bank, look for a common supply or connector. Low fault: P21EE; A injector high fault: P0289.

Cylinder 10 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light on
• Cylinder 10 weak or misfiring under load
• Lean mixture on the second bank

Most likely causes

• Cylinder 10 B injector control wire touching a live circuit
• Short inside a connector shared by several injectors
• Injector coil shorted
• Output stage overloaded by the short

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the connector and wiring of the injector indicated
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Measure the resistance and compare it with the manufacturer's specification
4. Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
5. On diesels, check the per-cylinder injection corrections

Good to know

• Repair difficulty: hard
• It affects emissions.

What each character of P21EF means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• EF: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21EF?

Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.

What causes code P21EF?

The most common causes are: cylinder 10 B injector control wire touching a live circuit; short inside a connector shared by several injectors; injector coil shorted.

Related codes

• P21EECylinder 10 Injector B Circuit Low
• P21D8Cylinder 10 Injector B Circuit/Open
• P0289Cylinder 10 Injector A Circuit High
• P21EDCylinder 9 Injector B Circuit High
• P21F1Cylinder 11 Injector B Circuit High
• P0310Cylinder 10 Misfire Detected

Sources

• gsi.ext.gm.com: 19OBDG03A Diagnostics
• en.wikipedia.org: Lamborghini Huracán
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.