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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P202C

P202CReductant Tank Heater Control Circuit High

P202C means the engine control module (ECM) has detected an abnormally high voltage on the control circuit for the reductant tank heater, which is the heating element that keeps diesel exhaust fluid (DEF) from freezing in cold weather. The ECM commands the heater on or off and expects the circuit voltage to respond within a normal range; a high-voltage reading tells the ECM that the circuit is open, shorted to power, or that the heater element itself has failed.

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
  • A driver information message warning that the DEF system is disabled or that the vehicle will not restart after shutdown
  • Reduced engine power or a speed-limiting strategy in some vehicles once the fault is active
  • The DEF tank heater may not activate in freezing temperatures, leading to a frozen reductant tank and a no-start condition
  • Possible stored freeze-frame data showing low ambient temperatures when the fault was set

Common causes, most likely first

  1. Open circuit in the reductant tank heater control wire, such as a broken wire or corroded connector terminal
  2. Failed reductant tank heater element (open heating coil inside the tank)
  3. Short to battery voltage on the heater control circuit, often from chafed wiring harness insulation
  4. Faulty reductant tank heater relay or control module, depending on the vehicle architecture
  5. Corroded or water-intruded electrical connector at the DEF tank or heater module
  6. Internal driver failure inside the ECM or dedicated reductant control module

How to diagnose and fix it

  1. Scan all modules for stored and pending codes, then record freeze-frame data to understand the conditions when P202C set
  2. Visually inspect the reductant tank heater wiring harness and connectors for chafing, corrosion, or water intrusion
  3. Measure resistance of the reductant tank heater element with a multimeter and compare to the manufacturer's specification
  4. Check for battery voltage on the heater control circuit with the key on and the heater commanded off, looking for an unexpected short to power
  5. Perform a voltage drop test on the heater ground circuit and the power feed to confirm the circuit can carry current
  6. Command the reductant tank heater on with a scan tool and verify the control circuit voltage toggles as expected
  7. If the circuit and heater element test good, substitute a known-good relay or control module and recheck before condemning the ECM

What a high-voltage heater code really means

  • A high-voltage code on a heater control circuit most often indicates an open circuit rather than a short to power, because the ECM pulls the control line up and sees high voltage when no current flows through the heater element.
  • Technicians sometimes replace the DEF tank heater when the real fault is a single corroded terminal in a connector that is exposed to road splash and urea residue.
  • P202C can be set by a weak or failing battery if the heater circuit is tested during a low-voltage condition, so always verify battery health before chasing wiring.
  • On vehicles with a dedicated reductant control module, the high-voltage signal may originate from a failed driver inside that module rather than from the heater or harness.
  • The heater circuit is often only energized below a certain ambient temperature, so a fault may not reappear during a warm-weather test drive and can be missed during a quick diagnosis.

Can I keep driving with P202C?

You can usually drive the vehicle short distances to a repair shop, but avoid long trips or cold-weather operation because a disabled DEF heater can lead to a frozen tank and a no-restart condition. If the vehicle has entered a speed-limited or restart-prohibited strategy, stop driving and arrange service.

Tools that help

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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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