P21CF: Cylinder 1 Injector B Circuit/Open

P21CF flags an open drive circuit of the second injector on cylinder 1. Which of the two injectors is B depends on the maker: Ford gives the letter to the port injector and keeps A, coded P0201, for direct injection, whereas Mitsubishi calls its direct injectors B and files open circuits under the low code P21DB. Ford's 2020 to 2025 gasoline summaries test the B circuits continuously above 11 V and need 5 s. GM's 2019 LT5 V8 judges it in a separate injection module: over 200 kOhm on the switched-off low side in 20 of 25 samples, Type A with the MIL in the first trip. Ford bulletin SSM 49996 lists it with P21D0 and P21D1 on 2021 Bronco Sport 1.5 models with a harness damaged at the transmission. With direct injection still working, rough idle or P0301 is the usual sign.

Cylinder 1 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light on
• Rough idle or misfire on cylinder 1, often with P0301
• Further harness codes stored at the same time

Most likely causes

• Chafed or broken wire in the port injector harness (on the Bronco Sport near the transmission cover)
• Open coil in the cylinder 1 port injector
• Loose or corroded plug at the injector
• Faulty driver stage in the engine module or separate injector module

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 continuity of the wire between the component and the control module.
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: medium
• It affects emissions.

What each character of P21CF means

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

Frequently asked questions

Can I keep driving with code P21CF?

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 P21CF?

The most common causes are: chafed or broken wire in the port injector harness (on the Bronco Sport near the transmission cover); open coil in the cylinder 1 port injector; loose or corroded plug at the injector.

Related codes

• P0201Cylinder 1 Injector A Circuit/Open
• P21DBCylinder 1 Injector B Circuit Low
• P21DCCylinder 1 Injector B Circuit High
• P21D0Cylinder 2 Injector B Circuit/Open
• P21D1Cylinder 3 Injector B Circuit/Open
• P0301Cylinder 1 Misfire Detected

Sources

• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
• gsi.ext.gm.com: 19OBDG03A Diagnostics
• static.nhtsa.gov: MC 10200984 0001

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.