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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20C2

P20C2Reductant Heater C Control Circuit Performance

P20C2 indicates that the powertrain control module has detected a performance fault in the control circuit for the reductant heater C, which is one of the heating elements used to keep diesel exhaust fluid (DEF) from freezing and to bring it to operating temperature. The computer monitors the circuit for expected voltage or current behavior and sets this code when the heater does not respond as commanded, which can prevent the selective catalytic reduction system from dosing correctly.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • A driver information message warning that the DEF system is unavailable or that the vehicle will be speed-limited after a countdown
  • Longer than normal DEF thaw times in cold weather, which may delay SCR system readiness
  • Rough running or reduced engine power if the system enters a limp-home strategy
  • Increased DEF consumption or failure to meet emissions readiness monitors
  • In some vehicles, a no-start condition after the DEF system is disabled by the control module

Common causes, most likely first

  1. Failed reductant heater C element, typically an open circuit or shorted winding inside the heater
  2. Damaged or corroded wiring and connector terminals between the control module and the heater
  3. Blown fuse or failed relay that supplies power to the reductant heater circuit
  4. Faulty reductant heater control module or a driver circuit inside the powertrain control module
  5. DEF contamination or crystallization causing the heater to overheat or draw excessive current
  6. Poor ground connection for the heater circuit, leading to voltage drop under load
  7. Aftermarket or incorrect DEF fluid that freezes at a higher temperature and stresses the heater

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P20C2, then verify battery voltage and inspect the reductant heater fuse and relay.
  2. Visually inspect the heater C connector, terminals, and harness for corrosion, chafing, or pushed-back pins, and perform a wiggle test while monitoring heater circuit parameters.
  3. Measure resistance of the reductant heater C element with a multimeter and compare to the manufacturer's specification; an open or out-of-range reading confirms a failed heater.
  4. Check voltage and ground at the heater connector with the circuit commanded on; a missing supply or excessive ground-side voltage drop points to wiring or control module faults.
  5. Command the reductant heater on with a scan tool and observe current draw and temperature rise to confirm whether the heater responds and whether the control module is commanding it correctly.
  6. If the heater and wiring test good, substitute a known-good control module or perform pinpoint tests for the driver circuit as directed by the service information.
  7. After repair, clear codes, run the reductant heater and SCR system monitors to readiness, and verify no pending codes return during a road test.

What the reductant heater code really means

  • A P20C2 can be set by a heater that still works but draws slightly out-of-spec current, so resistance testing at the connector is more reliable than assuming the heater is dead.
  • Corroded terminals at the heater connector are a frequent cause on vehicles that see road salt, and the corrosion often hides behind the connector seal where a visual inspection misses it.
  • The code does not always mean the heater element itself has failed; a weak battery or charging system can cause the control module to disable the heater and set a performance code.
  • Some vehicles set P20C2 after a DEF quality fault or a frozen DEF tank, so always check for related codes and verify the DEF is correct before replacing the heater.
  • A failed reductant heater can cause the SCR system to stop dosing, which may lead to a separate NOx efficiency code later if the vehicle is driven for an extended period.

Can I keep driving with P20C2?

You can usually drive the vehicle short distances to a repair shop, but avoid long trips or cold-weather operation because the DEF system may become unavailable and the vehicle may eventually limit speed or refuse to restart. If the dashboard displays a countdown or a speed limitation warning, have the fault repaired before the countdown expires.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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