Fault inside the on-board charger, such as failed power stage or internal sensor error; Poor or low-voltage supply from the charging point, cable or home supply; Damaged, corroded or loose connections in the charge port or high-voltage charging cables; Weak or discharged 12 V battery, or poor power or earth feeds to the charger control circuit; High-voltage battery fault, such as imbalanced or degraded cells or a contactor problem, causing the charger to limit output; Charger overheating or cooling system fault causing the output to be limited; Outdated or faulty control unit software, where the manufacturer has identified this as a known cause.

P0D21

Battery Charger A Output Voltage Too Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
D: AreaReserved
21: FaultThe specific fault in that area

What it means

P0D21 means the control unit that manages the on-board battery charger (charger A) saw the voltage it delivers to the high-voltage battery come out lower than the target for the current charging stage. It is set during a charge on a plug-in hybrid or electric vehicle. The exact thresholds and the way the charger is monitored depend on the manufacturer and model.

Symptoms

Car will not charge, or charges much more slowly than normal
Charging starts then stops part-way through
Charging fault or HV system warning light on the dashboard
Reduced battery charge level or electric range
Charge port indicator showing a fault or flashing an error pattern
Engine runs more often than usual on a plug-in hybrid because the battery is not charging

Possible causes

Fault inside the on-board charger, such as failed power stage or internal sensor errorlikely
Poor or low-voltage supply from the charging point, cable or home supplylikely
Damaged, corroded or loose connections in the charge port or high-voltage charging cablespossible
Weak or discharged 12 V battery, or poor power or earth feeds to the charger control circuitpossible
High-voltage battery fault, such as imbalanced or degraded cells or a contactor problem, causing the charger to limit outputpossible
Charger overheating or cooling system fault causing the output to be limitedpossible
Outdated or faulty control unit software, where the manufacturer has identified this as a known causerare

How to diagnose

Read all stored and pending codes and freeze frame data, noting the charge state, supply voltage, temperature and battery voltage when the fault set
Check for other related codes (HV battery, charger communication, 12 V supply, cooling) and deal with those first
Try a different charging point and a different cable, and confirm the home or public supply voltage is stable and correct
Check the 12 V battery condition and charging system, and check the charger's fuses and relays where fitted
Visually inspect the charge port, pins and cable for damage, water, corrosion or heat marks, and check accessible low-voltage connectors at the charger
Compare live data for charger output voltage and current against HV battery pack voltage and cell balance during a charge
Check charger cooling: coolant level, pump operation and airflow, following the manufacturer's procedure
Have a technician trained in high-voltage systems isolate the system and test the HV cables, connections and insulation resistance, and check for any software update the manufacturer has issued for this fault

How to fix

Repair or replace damaged charge port, connectors or HV charging cables
Correct the supply problem by repairing the charging point or cable, or the home installation
Replace a weak 12 V battery and repair any power or earth faults to the charger
Repair or replace the on-board charger if testing confirms an internal fault
Repair the cooling system fault, then recheck charger temperatures
Service or repair the HV battery fault found, or apply a manufacturer software update if one is specified for this fault

Can I keep driving?

Medium severity

If the car otherwise drives normally, it can usually be used, though range or charging will be limited, so book a diagnosis soon. If there is a red HV warning, loss of drive power or a burning smell, stop safely and have it recovered. High-voltage components must only be worked on by trained technicians.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.