P21C2: Reductant Heater Relay Control Circuit/Open

P21C2 concerns the control line of the relay that supplies current to the DEF heaters. The control module drives the relay coil through an output stage and checks whether the circuit reacts; an open circuit means the coil, its wiring or a connector has lost continuity, so the heaters cannot be energised. Ford documents the code in its 2023 diesel summary: the check runs continuously while heating is requested, and with the relay stage on and every heater switched off, a heater current below 0.2 A sets P21C2 within at most 5 s. P21C3 and P21C4 are the low and high faults of the same line, P21C7 covers the power relay of the module itself. In frost the fluid may stay frozen, so dosing starts late and DEF warnings can follow.

Symptoms

• Check engine light on
• DEF system warning, especially at low ambient temperature
• Frozen DEF thaws slowly, dosing begins late after a cold start

Most likely causes

• Relay coil burnt open or relay not seated in its socket
• Broken control wire between module and relay
• Corroded or loose terminals at relay box or module connector
• Faulty output stage in the controlling module

What to check, in order

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

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Check the continuity of the wire between the component and the control module.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P21C2 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• C2: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21C2?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P21C2?

The most common causes are: relay coil burnt open or relay not seated in its socket; broken control wire between module and relay; corroded or loose terminals at relay box or module connector.

Related codes

• P21C3Reductant Heater Relay Control Circuit Low
• P21C4Reductant Heater Relay Control Circuit High
• P21C7Reductant Control Module Power Relay/Relays Control Circuit/Open
• P20B9Reductant Heater A Control Circuit/Open
• P21DDReductant Heater A Current Too Low

Sources

• fordservicecontent.com: obd summary diesel 2023
• law.cornell.edu: 13 CCR 1968.2

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.