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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20FB

P20FBReductant Pump B Control Performance/Stuck Off

P20FB means the engine control module (ECM) has commanded the diesel exhaust fluid (DEF) reductant pump B to run, but the pump circuit or pump itself is not responding as expected, indicating a stuck-off or performance fault. The ECM typically detects this by monitoring pump current, pressure feedback, or a lack of expected pressure rise when the pump is energized.

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

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) with no other noticeable symptoms
  • Reduced engine power or limp mode due to emissions system derate
  • A message on the driver information center indicating low DEF, incorrect DEF, or service emissions system
  • Rough running or hesitation if the pump failure affects reductant dosing and triggers a derate
  • In some cases, the vehicle may enter a countdown to speed limitation if the fault is not addressed

Common causes, most likely first

  1. Failed reductant pump B (open winding, seized motor, or internal short)
  2. Blown fuse or damaged relay supplying power to the reductant pump circuit
  3. Corroded, loose, or damaged wiring or connector at the pump or ECM
  4. Restricted or blocked DEF filter or pickup screen causing the pump to run dry or overheat
  5. Frozen or crystallized DEF in the pump or lines (especially in cold climates)
  6. Faulty pump control module or driver circuit inside the ECM
  7. Low DEF level or incorrect fluid causing the pump to cavitate and fail performance checks

How to diagnose and fix it

  1. Check for any related technical service bulletins (TSBs) or software updates for the reductant pump control system
  2. Verify DEF level and condition; top off with fresh DEF if low or contaminated
  3. Inspect the reductant pump fuse and relay; replace if blown or faulty
  4. Perform a visual inspection of the pump connector and wiring for corrosion, chafing, or loose pins
  5. Use a scan tool to command the reductant pump on and observe live data for current draw and pressure response
  6. Test the pump motor directly with a known-good power source to confirm it operates
  7. If the pump fails to respond, replace the reductant pump assembly and clear the code

Reductant pump B stuck-off diagnosis tips

  • A stuck-off pump can be caused by a simple blown fuse that is often overlooked because the fuse box diagram may not clearly label the reductant pump circuit.
  • Crystallized DEF from a vehicle that sat unused for extended periods can seize the pump, mimicking an electrical failure; always inspect for white crusty deposits before condemning the pump.
  • The ECM may set P20FB even when the pump runs but the pressure sensor feedback is faulty, so verifying actual pressure with a mechanical gauge is a crucial step.
  • After replacing the pump, some vehicles require a relearn or adaptation procedure with a scan tool to prevent the code from returning immediately.
  • A weak battery or low charging system voltage can cause the pump to draw excessive current and set a performance code without any hard failure.

Can I keep driving with P20FB?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads or long trips because the emissions system may enter a derate strategy that limits speed and power. If the check engine light begins flashing or a countdown to speed limitation appears, stop driving and seek service immediately.

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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