Powertrain, generic (extended)

P20B4 Reductant Heater Coolant Control Valve Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P20B4 is the opposite electrical case to P20B3: the drive circuit of the reductant heater coolant valve reads too high, which points to a short to battery voltage on the control line or, with certain driver designs, an interrupted ground return. The control unit can then no longer switch the valve, and the heating loop stays in whatever position the valve spring holds. On coolant-heated installations, such as Bosch supply modules supplied with a cooling-water heating option, that decides whether the module thaws at all. No manufacturer documents P20B4 publicly; CARB's comprehensive component rules form the basis. Measure with the valve connected, because some controllers cut the output after a fault, as Hino describes for its valve.

Likely causes

Valve control wire shorted to battery voltage
Open ground return of the valve circuit
Output stage in the control unit defective

Symptoms

  • Malfunction indicator and SCR warning on
  • Valve cannot be actuated in an output test
  • Slow DEF thaw in winter

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–800
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

P20B1P20B2P20B3

References: law.cornell.edu, boschaftermarket.com, static.nhtsa.gov.