01 P0068 - Throttle Body Airflow Performance (PCM) • 02 Mastering the Art of Speed Shifting: Unveiling the Techniqu… • 03 How do driverless cars navigate without road markings? • 04 How to Troubleshoot and Repair Common Powertrain DTC Codes … • 05 Unveiling the Mystery of Gaskets: Your Key to Automotive Se… • 06 U0011 Medium Speed CAN Communication Bus Performance • 07 From Manifold to Tailpipe: How Exhaust System Components Bo… • 08 P0065, P0066, and P0067: Air Assisted Injector Control • 09 Engine Timing: A Comprehensive Guide for Successful Adjustm… • 10 Avoid These Brake Pads If You Drive in Rainy or Coastal Reg… •

DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05ED

P05EDReductant Heater Control Module Supply Voltage

P05ED indicates that the reductant heater control module has detected an abnormal supply voltage condition, either too low or too high, on its power feed circuit. The engine control module sets this code when the voltage at the heater control module's supply input falls outside the manufacturer's expected range for a calibrated time, preventing reliable reductant heating.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard
  • Often no symptoms besides the warning light, especially in warm ambient temperatures where reductant heating is not required
  • Possible reduction in engine power or limp mode if the system cannot maintain reductant temperature
  • Longer than normal reductant thaw times in freezing conditions, leading to delayed selective catalytic reduction (SCR) operation
  • A stored freeze-frame showing low or high system voltage at the time of fault detection
  • In some vehicles, a message indicating incorrect diesel exhaust fluid (DEF) quality or a service emissions system warning

Common causes, most likely first

  1. Weak or failing vehicle battery, causing supply voltage to drop below the module's threshold during cranking or high electrical load
  2. Corroded, loose, or damaged wiring and connectors in the power supply circuit to the reductant heater control module
  3. Blown fuse or faulty relay that feeds the reductant heater control module
  4. High resistance in the ground circuit for the heater control module, causing voltage drops under load
  5. Internal fault in the reductant heater control module causing it to draw excessive current or misreport supply voltage
  6. Charging system fault, such as a failing alternator, producing overvoltage that exceeds the module's safe operating limit
  7. Aftermarket electrical accessories or improper jump-starting that introduce voltage spikes into the supply circuit

How to diagnose and fix it

  1. Perform a battery and charging system test, including a load test and alternator output check, to rule out low or high supply voltage at the source
  2. Inspect the reductant heater control module power supply wiring and connectors for corrosion, chafing, loose pins, or heat damage, and repair as needed
  3. Check the fuse and relay that supply power to the reductant heater control module, replacing any that are blown or intermittent
  4. Measure voltage drop on both the positive supply and ground circuits under load to identify excessive resistance in the feed or return path
  5. Clear the code and perform a road test or actuation test of the reductant heater to confirm the fault returns under the same conditions
  6. If voltage supply is confirmed good and the code persists, test the reductant heater control module according to the manufacturer's diagnostic procedure, which may involve back-probing the module connector or using a scan tool bidirectional control
  7. Replace the reductant heater control module only after all external power and ground circuits are verified good, then clear codes and verify repair

Supply voltage faults are rarely the module itself

  • A large percentage of P05ED cases are caused by a weak battery or poor connection rather than a failed heater control module, so always load-test the battery and check voltage drop before condemning parts.
  • The code can set intermittently during cold starts when the glow plugs and other high-current consumers pull the battery voltage down, even if the battery passes a static test.
  • Corrosion inside a connector that looks fine from the outside can create enough resistance to drop the supply voltage only when the heater is commanded on, so wiggle-test and back-probe under load.
  • Jump-starting another vehicle or being jump-started incorrectly can spike the supply voltage and set this code without any permanent damage, so clear it and retest before replacing anything.
  • Some vehicles set P05ED when the reductant heater control module supply voltage is high due to a faulty voltage regulator in the alternator, which is often overlooked because the focus stays on low-voltage causes.

Can I keep driving with P05ED?

You can usually continue driving with P05ED, but avoid long trips in freezing weather because the reductant may not thaw properly, which can lead to reduced engine power or a no-start condition. Have the charging system and wiring inspected soon to prevent the fault from worsening.

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.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage