P21CA: Reductant Control Module Supply Voltage Circuit

What it means

A fault has been detected in the reductant control module supply voltage circuit. The code does not establish that the module itself diagnosed a supply-pin fault.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The reductant control module needs a stable electrical supply to control DEF delivery for exhaust emissions treatment. P21CA indicates a fault in the module's supply voltage circuit, but does not establish that the module itself identified a bad supply pin.

Common causes

Most likely first.

1. Open or high resistance in module power wiring
2. Blown fuse or poor fuse connection
3. Loose or corroded power connector
4. Poor ground connection

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Record freeze-frame data and scan for battery, charging, and other reductant-system codes to check whether the supply fault is part of a wider electrical problem.
2. Inspect the module power and ground connectors for looseness, corrosion, moisture, or damaged wiring.
3. Check the relevant fuse and its terminals for an open fuse or poor contact.
4. Measure voltage at the module power and ground circuits under the conditions specified by the manufacturer, comparing readings with the manufacturer's specification.
5. If supply is still incorrect, test wiring continuity and voltage drop between the battery, fuse, grounds, and module to locate an open or high-resistance connection.

Common fixes

• Clean and secure a loose or corroded power or ground connection.
• Replace a blown fuse after checking for the fault that caused it to open.
• Repair an open or high-resistance power or ground wire.
• Repair or replace a damaged module connector.

Can I keep driving?

You may be able to drive at first, but the reductant system can stop treating exhaust and may start a no-start countdown after the ignore period. Limit unnecessary trips and have the supply fault fixed before the countdown expires.