P20FB: Reductant Pump B Control Performance/Stuck Off
What to do
Ask whether this vehicle has two dosing points or a transfer pump feeding a smaller tank, since the same code means different things depending on the layout and it decides what gets checked.
Ask how the other pump is behaving. If the first pump is working normally and only this one is not, the fault is local to it — but if both are struggling, the cause is usually the fluid or a shared blockage, and no pump needs replacing.
Mention the weather if it is cold, since partly frozen fluid stops a pump delivering with nothing broken.
On a working vehicle, be aware it may seem fine unladen and still fail its emissions target under load, so do not judge by how it drives.
What this code means
The second reductant pump is not delivering when commanded, or is stuck off.
A second reductant pump means a larger system than a passenger car's, and identifying what it serves is the first step. Two arrangements are common: a heavy vehicle with two dosing points — one before each of two catalyst sections — or a system where a transfer pump moves fluid from a main tank to a smaller dosing tank while a second pump does the metering.
Those are different enough that the same code means different things, so establish which this vehicle has before diagnosing.
The causes are its sibling's: crystallisation, a blocked filter, frozen fluid, a worn pump, or an electrical fault upstream.
What a second pump adds is a control, and it is a good one. Pump A and pump B do the same job with the same fluid from the same tank. If A is working and B is not, the fault is local to B — its wiring, its filter, its own crystallisation. If both are struggling, the fluid or the shared supply is the problem, and no pump needs replacing.
That comparison is worth making explicitly before ordering a part, because these pumps are not cheap and the shared causes — frozen fluid, contaminated fluid, a blocked main filter — are much cheaper to put right.
On commercial vehicles, note that dosing capacity is sized for the duty. A vehicle running with one pump down may appear fine unladen and fail its emissions target under load.
Likely causes
Start here
- Crystallised reductant injector · moderate
- Faulty reductant pump · moderate
- Blocked reductant filter · moderate
Less common
- Frozen reductant in the tank or lines · moderate
- Corroded or loose connector · easy
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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