P21CC code meaning
Reductant Control Module Supply Voltage High
Powertrain system Generic code P21xx codes Check engine light
What P21CC means
P21CC is the overvoltage counterpart of P21CB: the DEF control module sees a supply above what its electronics are designed for. Ford's 2023 diesel summary checks the reductant pump controller in a continuous loop and sets P21CC once its supply exceeds 19 V, deciding within 5 s. On a 12 V vehicle such values usually come from a charging system whose regulator has failed, from a jump start with a 24 V source, or from load dumps on a loose battery terminal; less often the module measures its own input wrongly. To protect pump and heaters the module usually stops working, so dosing pauses. GM's published diesel tables list only the low case. P21CA is the general supply code, and P0563 shows whether the engine module sees the same excess.
P21CC at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor0.3 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P21CC
- Check engine light on
- DEF dosing paused, SCR warning
- Battery or charging warning, bulbs unusually bright
Common causes of P21CC
- Most likelyAlternator voltage regulator overcharging
- PossibleJump start or charging with excessive voltage
- PossibleLoose battery terminal causing voltage spikes
- Less likelyModule's own voltage measurement faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
SCR control module
What each part of P21CC means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 1 | Area: fuel and air metering. |
| CC | The exact fault in that area. |
What to do next
- Note P21CC and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P21CC
What does P21CC mean?
P21CC means "Reductant Control Module Supply Voltage High". P21CC is the overvoltage counterpart of P21CB: the DEF control module sees a supply above what its electronics are designed for. It is a generic code, so it means the same on every make.
Is it safe to drive with P21CC?
Drive gently and only to a workshop. With P21CC the car can go into limp mode (reduced power) to protect itself.
Will P21CC fail an emissions test?
Yes, it can. P21CC is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P21CC to fix?
Difficulty: medium. A typical repair takes 0.3 to 2.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P21CC clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.