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 a part of the car that helps clean the exhaust. It's like a heater that isn't working right, and it makes the car unhappy.
The P20C4 code is triggered when the Engine Control Module (ECM) detects a high voltage condition in the control circuit of the reductant heater C.
The reductant heater is part of the Selective Catalytic Reduction (SCR) system, which reduces nitrogen oxide emissions in diesel engines. Heater C is responsible for maintaining the proper temperature of the DEF (Diesel Exhaust Fluid) to ensure it flows correctly. A high voltage condition typically indicates an electrical fault such as a short circuit or a failed component.
• Check Engine Light illuminated
• Possible decrease in fuel efficiency
• Increased exhaust emissions
• Faulty reductant heater C
• Open or shorted wiring in the heater circuit
• Faulty ECM or control module
ECM: Engine Control Module
DEF: Diesel Exhaust Fluid
SCR: Selective Catalytic Reduction
• Use an OBD-II scanner to confirm the code.
• Inspect the wiring and connectors for damage or corrosion.
• Test the reductant heater C for proper resistance and operation.
• Repair or replace damaged wiring or connectors.
• Replace the reductant heater C if faulty.
• Update or reprogram the ECM if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $500
Moderate
• Ford F-250 Super Duty
• Chevrolet Silverado 2500HD
• Ram 2500
• Regular maintenance of the SCR system
• Ensure DEF is of good quality and not contaminated
• Inspect wiring and connectors periodically
Sensor Issue
• Do not ignore the Check Engine Light.
• Have the vehicle inspected by a professional if unsure.
Repairs may be covered under emissions warranty; check with the manufacturer.
This code has become more common with the increased use of SCR systems in diesel engines to meet emissions standards.
Moderate – While the vehicle may still operate, emissions and fuel efficiency could be affected.
Driving with a malfunctioning emissions system may violate emissions laws and contribute to environmental pollution.
• Engine running
• DEF system active
The reductant heater is located in the DEF tank or along the DEF line, depending on the vehicle design.
Q: Can I drive with a P20C4 code?
A: You can drive, but it's not recommended as it may affect emissions and fuel efficiency.
Q: What happens if I ignore this code?
A: Ignoring the code can lead to increased emissions, potential engine performance issues, and may fail emissions tests.
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