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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20ED

P20EDReductant Heater "A" Control Stuck On

P20ED means the engine control module has commanded the reductant heater 'A' circuit off, but the circuit feedback indicates it is still drawing current or staying energized. The computer detects this as a stuck-on condition because the heater control driver sees voltage where it should be zero after the command is removed.

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

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • Diesel exhaust fluid (DEF) system warning message on the dashboard
  • Possible reduction in engine power or limp mode if the fault persists
  • Longer than normal DEF thaw time in freezing temperatures
  • Overheating or burning smell from the reductant heater area in severe cases

Common causes, most likely first

  1. Short to voltage in the reductant heater control wiring
  2. Failed reductant heater relay or control module stuck closed
  3. Internal short in the reductant heater element itself
  4. Corroded or damaged connector pins causing a false feedback signal
  5. Faulty engine control module driver circuit
  6. Aftermarket or incorrect heater component installed

How to diagnose and fix it

  1. Scan all modules for related codes and record freeze-frame data to confirm the command and feedback states
  2. Inspect the reductant heater wiring harness for chafing, pinched sections, or signs of overheating
  3. Disconnect the heater connector and measure resistance across the heater element to check for an internal short
  4. Test the heater control relay or driver circuit for continuity and proper switching with a multimeter
  5. Check for voltage on the control circuit with the key on and the command off to identify a short to power
  6. Substitute a known-good relay or control module and clear codes to see if the fault returns
  7. If wiring and components test good, perform a guided diagnostic for the engine control module driver circuit

What a stuck-on heater really means

  • A stuck-on heater can drain the battery overnight because the element continues to draw current even with the key off.
  • Many technicians replace the heater element first, but a shorted control wire or relay is just as common and far cheaper to fix.
  • The code can set intermittently from water intrusion into a connector, so always check for green corrosion before condemning parts.
  • A reductant heater that stays on can overheat the DEF tank and damage the fluid, leading to secondary quality codes.
  • Some vehicles use a pulse-width-modulated driver, so a stuck-on fault may only appear when the command is at zero percent duty cycle.

Can I keep driving with P20ED?

You can usually drive with this code, but avoid long trips or leaving the vehicle parked for extended periods until it is repaired, because a stuck heater can drain the battery and overheat the DEF system. If the engine enters limp mode or you smell burning, stop driving and have it serviced.

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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