What the SPN 4354 Code Means

SPN 4354 refers to the DEF Line Heater 1 circuit — the main heated hose that carries AdBlue from the tank to the injector. Its job is to stop the fluid freezing in temperatures below 10°C. If the ECU detects open resistance or low current draw, it logs the code and may trigger a fault light or countdown.

Most commonly, the associated FMI codes explain what failed:

• FMI 5 – Open Circuit (no continuity in heater wiring)
• FMI 6 – Short to Ground or Battery
• FMI 18 – Current Below Normal (weak element or corroded connector)

Vehicles Affected

This fault regularly appears on Mercedes Sprinter, VW Crafter, Ford Transit, and Peugeot Boxer vans — all of which share similar SCR dosing layouts. In cold conditions, the heater works hard to keep DEF fluid above its freezing point, which eventually causes internal wear.

Common Symptoms

• “AdBlue System Fault” or “DEF Heater Malfunction” warning
• SPN 4354 logged alongside P20BA or P207F
• Slow AdBlue dosing during cold starts
• Countdown warning “No Start in 500 miles”
• DEF pump whine or delayed pressure build-up

How to Diagnose SPN 4354

Using a professional scan tool, check live current draw for the line heater. A healthy circuit typically draws 3–4 amps at 12V. Anything below 2A usually points to corrosion or partial internal failure.

Next, visually inspect the DEF line connector. Green or white residue near the pins shows water ingress, which increases resistance. Repairing the wiring or fitting a new connector often clears the fault permanently.

How Our Mobile Technicians Fix It

At AdBlue Specialist, we start every SPN 4354 callout with live electrical tests and a software reset to confirm whether the heater circuit is open or weak. If the wiring is intact, we can safely bypass the failed element or apply an ECU-level heater delete for off-road or export vehicles.

This method stops countdown warnings while keeping the SCR system stable — no resistors, no dashboard tampering, and no risk to the ECU. All work is carried out through the OBD port at your location.

How It Links to Other Codes

SPN 4354 is often found with:

• P207F – DEF Quality Performance
• P20BA – Reductant Heater Circuit A
• P3041 – Heated Pipe Fault

In most cases, clearing SPN 4354 after repair will also remove these related codes once the ECU detects stable dosing temperature.

When It Happens Most

Heater faults typically appear after the first cold snap of the year. Vans used for short city runs are more likely to trigger SPN 4354 because the SCR system rarely reaches full operating temperature, causing moisture to build up inside the line. Fleet vehicles parked outside overnight are particularly at risk.

Prevention Tips

• Run the engine for at least 20 minutes weekly in winter to keep DEF lines warm.
• Check harness connectors near the tank and pump for corrosion.
• Never power-wash the AdBlue tank or undertray during freezing weather.
• Book an annual SCR inspection before winter to test heater resistance.

Mobile Fix vs Workshop Visit

Dealerships often quote full tank replacements for SPN 4354 faults, which can cost £900–£1,200. In most cases, only the heated hose or connector needs repair. Our mobile AdBlue repair service can fix the issue for a fraction of that cost — often within an hour.

Is It Safe to Keep Driving?

Yes, temporarily. The system will continue dosing while temperatures stay above freezing. But once the line freezes overnight, the SCR system shuts down and the countdown begins. It’s best to get it fixed before that happens.

Why Choose AdBlue Specialist

• 250+ AdBlue and SCR system faults fixed since 2021
• Mobile service across Staffordshire and Cheshire East
• Certified, insured ECU software technicians
• Fast, safe, MOT-friendly repairs — no cutting or drilling

Need Help With an SPN 4354 Fault?

If your van shows SPN 4354 or P20BA on the dash, we can help the same day. Our mobile team can test, repair, or safely reset the AdBlue heater circuit wherever you are.