Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means there's a problem with the part of the car that heats up a special liquid (called reductant) used to make the exhaust cleaner. It's like when a heater in your house doesn't turn on because of a broken switch.
The P21C2 code indicates that the vehicle's computer has detected an issue with the circuit that controls the relay for the reductant heater. This could mean the circuit is open or not functioning properly.
The reductant heater is responsible for heating the diesel exhaust fluid (DEF) to ensure it remains at an optimal temperature for proper emission control. The relay control circuit is monitored by the engine control module (ECM), and an open circuit or failure in this relay can trigger the P21C2 code.
• Check Engine Light is on
• Reduced engine performance
• Increased exhaust emissions
• Possible DEF warning light
• Faulty reductant heater relay
• Open or shorted wiring in the reductant heater circuit
• Damaged connectors or terminals
• Faulty engine control module (ECM)
Reductant: A liquid, often diesel exhaust fluid (DEF), used to reduce emissions from diesel engines.
Relay: An electrical switch that opens and closes under the control of another electrical circuit.
• Use an OBD-II scanner to confirm the code P21C2.
• Inspect the wiring and connectors related to the reductant heater relay for damage or corrosion.
• Test the reductant heater relay for proper operation.
• Check the ECM for any faults or malfunctions.
• Replace the reductant heater relay if faulty.
• Repair or replace damaged wiring or connectors.
• Update or reprogram the ECM if necessary.
• Clear the code and test drive to ensure the issue is resolved.
• OBD-II scanner
• Multimeter
• Basic hand tools (screwdrivers, pliers)
$100 - $300
Moderate
• Ford F-250
• Chevrolet Silverado
• Ram 2500
• Regularly inspect wiring and connectors for damage.
• Ensure the reductant heater system is serviced as per the vehicle's maintenance schedule.
• Use high-quality diesel exhaust fluid (DEF).
Emission Control System
• Do not ignore the Check Engine Light; address the issue promptly.
• Consult a professional mechanic if unsure about the repair process.
Check with your vehicle manufacturer for warranty coverage on emission-related components.
This code has become more common with the increased use of DEF systems in diesel vehicles to meet stricter emission standards.
Moderate – While the vehicle may still be drivable, addressing this issue promptly is important to maintain emission compliance and engine performance.
Operating a vehicle with a malfunctioning emission control system may violate environmental regulations.
• Engine at operating temperature
• Vehicle in a safe, stationary position
The reductant heater relay is typically located in or near the DEF tank or integrated within the vehicle's fuse box.
Q: Can I drive with a P21C2 code?
A: While the vehicle may still be drivable, it's recommended to fix the issue soon to avoid increased emissions and potential engine performance issues.
Q: What happens if I ignore the P21C2 code?
A: Ignoring the code can lead to increased emissions, potential damage to the emission control system, and possible legal penalties for non-compliance with emission standards.
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