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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20EC

P20ECReductant Heater "A" Control Stuck Off

P20EC means the engine control module has commanded the reductant heater circuit 'A' to turn on, but the circuit feedback indicates it is stuck off, so no current is flowing through the heater element. The computer detects this by monitoring the heater control circuit voltage or current and seeing no expected change when activation is requested.

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 pending or stored P20EC code with possible reduced engine power or limp mode
  • Longer diesel exhaust fluid (DEF) thaw times in cold weather, leading to delayed reductant injection
  • Increased DEF consumption or poor DEF dosing efficiency over time
  • Possible related codes for reductant heater performance or circuit faults

Common causes, most likely first

  1. Open or high-resistance wiring in the reductant heater control circuit, including broken wires or corroded connectors
  2. Failed reductant heater element (open circuit inside the heater)
  3. Blown fuse or faulty relay supplying power to the reductant heater circuit
  4. Poor ground connection for the heater circuit, causing no current flow
  5. Faulty engine control module (ECM) driver for the heater control, though rare
  6. Damaged or disconnected heater connector pins, often due to vibration or contamination

How to diagnose and fix it

  1. Check the reductant heater fuse and relay, replacing any blown fuse or faulty relay first
  2. Inspect the heater circuit wiring and connectors for damage, corrosion, or loose pins, and repair as needed
  3. Measure resistance of the reductant heater element with a multimeter and compare to specification; replace heater if open or out of range
  4. Test for voltage and ground at the heater connector when the heater is commanded on; if missing, trace back to the ECM or power source
  5. Check the ECM control signal at the heater driver output using a scan tool or oscilloscope to confirm command is present
  6. If all circuits and components test good, suspect a faulty ECM and perform further diagnostics before replacement
  7. Clear codes and verify repair by running the heater activation test with a scan tool and confirming no code returns

Reductant heater circuit checks that save time

  • A stuck-off heater often sets no drivability symptoms until cold weather exposes the lack of DEF thawing, so drivers may ignore it until a no-start or limp condition occurs.
  • The heater element can test good at room temperature but fail open when cold, so resistance checks should be repeated after cold-soaking the vehicle overnight.
  • Corrosion inside the heater connector is a frequent hidden cause that looks like a failed heater; always back-probe and inspect pins rather than trusting a visual check alone.
  • Some vehicles use a pulse-width modulated (PWM) signal to control the heater, so a simple test light may not confirm proper operation; a duty-cycle capable meter or oscilloscope is needed.
  • A weak or failing battery can cause the ECM to disable the heater circuit to protect voltage, setting P20EC even though the heater itself is fine.

Can I keep driving with P20EC?

You can generally continue driving with P20EC, but avoid long trips in freezing conditions because the DEF may not thaw, leading to reduced engine power or a no-start. Have the circuit inspected soon to prevent more severe reductant system faults.

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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