P20E8: Reductant Pressure Too Low
What to do
Look under the vehicle for white crystalline deposits around the reductant lines and connections. Reductant dries to a distinctive white crust, and finding it means a leak — which is both the easiest cause to confirm and useful to report before diagnosis starts.
Mention the weather. In cold conditions, partly frozen fluid produces low pressure with nothing actually broken — in which case the real fault is a heater, not the pump.
Mention roughly how often you top the tank up and whether that has changed, since a leak shows up as increased consumption.
Do not leave it. The system is under-dosing without any symptom you can feel, and the emissions sequence escalates towards a speed limit and eventually a restart lockout.
What this code means
The reductant system cannot reach the pressure it needs to dose accurately.
Pressure is what turns an injector opening time into a known quantity of fluid. The pump holds the line at several bar; the module then meters by how long the injector opens. If pressure is low, every dose is smaller than the module believes — so the system under-doses while thinking it is doing the right thing, and NOx output rises without any obvious symptom.
The causes fall into three groups, and they are separated by when the fault appears.
- A restriction on the supply side — a blocked filter, a crystallised line, or a pump strainer clogged with deposits. This worsens gradually.
- A leak. Reductant escaping between pump and injector, which leaves the unmistakable white crystalline residue that makes this the easiest cause to confirm by looking.
- A pump that can no longer build pressure, from wear or from crystallisation inside it.
Cold weather deserves separate consideration and is the seasonal answer. Partially frozen fluid, or slush in the lines, produces low pressure with entirely sound hardware — so a low-pressure code in winter should prompt a check of the heater circuits before anything is replaced.
Look underneath for white deposits before anything is dismantled. It takes a minute and it either finds the fault or rules out the easiest cause.
Likely causes
Start here
- Reductant leak from tank or lines · moderate
- Blocked reductant filter · moderate
- Faulty reductant pump · moderate
Less common
- Frozen reductant in the tank or lines · moderate
- Crystallised reductant injector · moderate
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
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
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.
Sign in or create an account to add what you found about P20E8.