Powertrain, generic (extended)

P20B3 Reductant Heater Coolant Control Valve Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P20B3 reports a low reading on the control line of the coolant valve that feeds the DEF heating, the classic picture of a control wire shorted to ground or a solenoid with shorted turns. Depending on how the output stage is wired, the valve then stays permanently energised or never responds, so the DEF can get too warm in summer or remain frozen in winter. No manufacturer is known to describe P20B3 publicly; the classification rests on the CARB rule that actuators in the SCR system be checked continuously for circuit faults. The valve exists mainly on coolant-heated systems, where it governs how quickly frozen DEF becomes available. Before replacing it, look for chafed wiring along the frame and for coolant or DEF inside the connector.

Likely causes

Valve control wire shorted to ground
Valve solenoid with shorted turns (low resistance)
Coolant or DEF inside the valve connector

Symptoms

  • Malfunction indicator and SCR warning on
  • DEF tank overheats or does not thaw, depending on valve position

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–600
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant coolant valve

Related codes

P20B1P20B2P20B4

References: law.cornell.edu, en.wikipedia.org, boschaftermarket.com.