P21EC: Cylinder 9 Injector B Circuit Low

P21EC signals a low circuit state, usually a grounded wire or, depending on the maker, also a break, on the B injector of cylinder 9. It can only occur on an engine with at least nine cylinders and two injectors per cylinder. No manufacturer documents it publicly; GM's and Ford's lists stop at cylinder 8. The one such engine on record is Lamborghini's Huracán V10, which combines direct and multi-point injection; which of the two it calls B is not published. Its firing order alternates between cylinders 1 to 5 and 6 to 10, so cylinder 9 belongs to the second bank. CARB requires circuit monitoring of such output components. By analogy with GM's V8, a ground below about 0.5 Ohm on the switched-off low side would count. Open circuit: P21D7; high fault: P21ED.

Cylinder 9 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light on
• Misfire or rough running on cylinder 9
• Mixture deviation on the bank of cylinder 9

Most likely causes

• Cylinder 9 B injector wire shorted to ground
• Broken wire, if the maker reports opens under the low code
• Injector coil shorted to housing or open
• Engine control module output faulty

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. With the connector unplugged, check whether the signal wire is making contact with ground.
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 P21EC means

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

Frequently asked questions

Can I keep driving with code P21EC?

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 P21EC?

The most common causes are: cylinder 9 B injector wire shorted to ground; broken wire, if the maker reports opens under the low code; injector coil shorted to housing or open.

Related codes

• P21EDCylinder 9 Injector B Circuit High
• P21D7Cylinder 9 Injector B Circuit/Open
• P0285Cylinder 9 Injector A Circuit Low
• P21EACylinder 8 Injector B Circuit Low
• P21EECylinder 10 Injector B Circuit Low
• P0309Cylinder 9 Misfire Detected

Sources

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

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.