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 helps keep a special fluid warm. This fluid helps the car's exhaust be cleaner.
The P21C4 code indicates that the vehicle's computer has detected a high voltage condition in the circuit that controls the reductant heater relay.
The reductant heater is part of the diesel exhaust fluid (DEF) system, which helps reduce emissions in diesel engines. The relay controls the power to the heater. A 'circuit high' condition suggests that there is more voltage than expected, possibly due to a short circuit or a malfunctioning relay.
• Check Engine Light illuminated
• Possible reduction in engine performance
• Diesel exhaust fluid system warning light
• Increased emissions
• Faulty reductant heater relay
• Wiring issues such as shorts or open circuits
• Defective reductant heater
• Faulty powertrain control module (PCM)
Reductant: A fluid used in diesel engines to reduce emissions.
Relay: An electrical switch that controls a circuit.
• Use an OBD-II scanner to confirm the code.
• Inspect the wiring and connectors for damage or corrosion.
• Test the reductant heater relay for proper operation.
• Check the reductant heater for functionality.
• Replace the faulty reductant heater relay.
• Repair or replace damaged wiring or connectors.
• Replace the reductant heater if defective.
• Reprogram or replace the PCM if necessary.
• OBD-II scanner
• Multimeter
• Relay tester
• Basic hand tools
$100 - $500
Moderate
• Ford F-Series
• Chevrolet Silverado
• Ram 2500
• Regularly inspect and maintain the DEF system.
• Ensure connectors and wiring are free from corrosion.
• Use high-quality DEF fluid.
Sensor Issue
• Have the vehicle inspected by a professional mechanic.
• Avoid ignoring the warning light as it may lead to more severe issues.
Check with the vehicle manufacturer for specific warranty coverage related to emissions systems.
This code has become more common with the increased use of DEF systems in diesel vehicles.
Moderate – While the vehicle may still be drivable, addressing this issue promptly can prevent further emissions problems.
Ignoring emissions-related codes can lead to failing emissions tests and potential fines.
• Engine running
• DEF system operational
The reductant heater relay is typically located near the DEF tank or in the engine compartment fuse box.
Q: Can I drive with a P21C4 code?
A: It's possible to drive, but it's not recommended as it can lead to increased emissions and potential engine performance issues.
Q: How can I reset the P21C4 code?
A: After repairs, use an OBD-II scanner to clear the code. If the issue is fixed, it should not return.
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