P21E4: Cylinder 5 Injector B Circuit Low

P21E4 flags a low state on the circuit of the second (B) injector serving cylinder 5. The only public OBD summary that lists it is GM's for the 2019 Corvette ZR1, whose supercharged LT5 V8 pairs direct injectors with port injectors and drives the supplemental ones from a separate fuel injection control module (FICM). While that driver is off, a low side reading 0.5 Ohm or less toward ground in 20 of 25 samples, taken every 100 ms, is judged a short and becomes a Type A fault with the MIL in the first trip. On GM's V8 numbering cylinder 5 is the third cylinder of the left bank, i.e. bank 1. Because a grounded low side can let the injector flow uncommanded, a rich bank 1 may follow. Ford's dual-injection engines report B shorts on this cylinder only as P21D3.

Cylinder 5 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light on
• Rich running or fuel trim pulled back on bank 1
• Rough idle or misfire on cylinder 5

Most likely causes

• Low-side wire of the cylinder 5 B injector touching ground
• Injector winding shorted to its housing
• Moisture or corrosion in the injector connector
• Driver stage in the injection control module 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 P21E4 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• E4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21E4?

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 P21E4?

The most common causes are: low-side wire of the cylinder 5 B injector touching ground; injector winding shorted to its housing; moisture or corrosion in the injector connector.

Related codes

• P21E5Cylinder 5 Injector B Circuit High
• P21D3Cylinder 5 Injector B Circuit/Open
• P0273Cylinder 5 Injector A Circuit Low
• P21E2Cylinder 4 Injector B Circuit Low
• P21E6Cylinder 6 Injector B Circuit Low
• P0172System Too Rich Bank 1
• P0305Cylinder 5 Misfire Detected

Sources

• gsi.ext.gm.com: 19OBDG03A Diagnostics
• lsxmag.com: inside corvette lt5 engine
• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023

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.