P21DB: Cylinder 1 Injector B Circuit Low

P21DB means the drive circuit of the B injector on cylinder 1 is pulled toward ground. Two makers document it, with opposite meanings of B. Mitsubishi's 2022 Eclipse Cross uses B for the direct high-pressure injector (its port injector faults are P0261 ff.) and sets P21DB when an interruption or a grounded line persists for 2 s with 10 to 16.5 V battery and over 10 V on the injector driver supply. One faulty cylinder loses both its injectors; with several faults direct injection is switched off and the engine runs on port injection with limited torque. GM's 2019 LT5 V8 tests the low side of its supplemental B injector with the driver off: below 0.5 Ohm to ground in 20 of 25 samples, Type A. Ford covers B faults only with P21CF to P21D6, opens and shorts together. P21DC is the high counterpart, P21CF the open circuit.

Medium severity. Can I keep driving?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.

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

Symptoms

Check engine light on
Cylinder 1 misfire or shut-off
Reduced torque when several cylinders are affected (Mitsubishi)

Most likely causes

Injector wire chafed through to ground
Injector winding open or shorted to its housing
Damaged intermediate or injector connector
Engine or injection control module faulty

What to check, in order

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

Check the connector and wiring of the injector indicated
With the connector unplugged, check whether the signal wire is making contact with ground.
Measure the resistance and compare it with the manufacturer's specification
Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
On diesels, check the per-cylinder injection corrections

Good to know

Repair difficulty: hard
It can trigger limp mode.
It affects emissions.

What each character of P21DB means

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

Frequently asked questions

Can I keep driving with code P21DB?

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 P21DB?

The most common causes are: injector wire chafed through to ground; injector winding open or shorted to its housing; damaged intermediate or injector connector.

Related codes

Sources

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.