“AdBlue sensor fault – top up AdBlue” flashing on your dash, but you’ve just filled the tank? Or maybe the warning appeared after topping up, and now the van thinks the tank is empty even though you’ve added 10 litres. AdBlue sensor faults are one of the most common—and most frustrating—issues on modern diesel vans, because the symptoms don’t always match the actual problem. Your van has three main AdBlue sensors (level, pressure and temperature), and when one fails, the SCR system can’t tell if there’s enough fluid, if it’s being pumped correctly, or if it’s frozen. This guide explains how to diagnose which sensor has failed, what the fault codes mean, and when you can fix it yourself versus when you need a mobile specialist in Stoke-on-Trent.
The three AdBlue sensors (and what they do)
Modern AdBlue systems use three sensors to monitor fluid level, pressure and temperature. When one fails, the ECU can’t verify that the SCR system is working, so it triggers a countdown warning to force you to fix it before the engine locks.
1. AdBlue level sensor (quality sensor)
What it does: Measures how much AdBlue is in the tank using a float and ultrasonic sensor. Sends the reading to the dash display (“AdBlue range: 1,200 miles”) and triggers low-level warnings.
Common faults:
- Crystallised AdBlue on the sensor probe (reads “empty” even when full)
- Corroded wiring or connector (intermittent signal)
- Failed sensor electronics (stuck reading, no signal)
- Contaminated AdBlue (sediment blocks the ultrasonic beam)
Fault codes: P20EE (SCR NOx catalyst efficiency below threshold), P204F (reductant system performance), P207F (reductant level sensor circuit).
2. AdBlue pressure sensor
What it does: Monitors the pressure in the AdBlue delivery line between the pump and the dosing injector. Normal pressure is 5–9 bar. If pressure drops below 3 bar, the ECU assumes the pump has failed or there’s a leak.
Common faults:
- Blocked pressure sensor port (crystallised AdBlue)
- Failed sensor diaphragm (reads zero pressure even when pump is running)
- Air in the system (after running the tank dry or replacing the pump)
- Corroded connector or broken wiring
Fault codes: P20E8 (reductant pressure too low), P20BA (reductant pressure sensor circuit).
3. AdBlue temperature sensor
What it does: Monitors the temperature of the AdBlue in the tank and pump. The ECU uses this reading to activate the heater in cold weather (AdBlue freezes at -11°C) and to calculate the correct dosing rate (AdBlue viscosity changes with temperature).
Common faults:
- Failed heater circuit (sensor reads -40°C even in summer)
- Stuck reading (always shows 20°C, even when frozen)
- Corroded wiring or short circuit
Fault codes: P13DF (reductant heater control circuit), P20E9 (reductant temperature sensor circuit).
How to tell which sensor has failed
AdBlue sensor faults often cause similar symptoms (warning lights, countdown messages), but the fault code and dash behaviour tell you which sensor is the problem.
| Symptom | Likely Sensor | Fault Code |
|---|---|---|
| Dash shows “AdBlue empty” but tank is full | Level sensor | P20EE, P204F, P207F |
| Warning appeared immediately after topping up | Level sensor (contaminated or stuck) | P20EE, P204F |
| “AdBlue pressure too low” message | Pressure sensor or pump | P20E8, P20BA |
| Warning only appears in cold weather (below 0°C) | Temperature sensor or heater | P13DF, P20E9 |
| Countdown started after running tank dry | Level sensor or air in system | P20EE, P20E8 |
| Warning clears after restarting, then returns after 10–20 miles | Pressure sensor or intermittent wiring fault | P20E8, P20BA |
Pro tip: If you have multiple fault codes (e.g. P20EE + P20E8 + P204F), start with the oldest code first—that’s usually the root cause. The others are often secondary faults triggered by the first failure.
Step-by-step: How to diagnose an AdBlue sensor fault
You’ll need a diagnostic scanner (OBD2 reader with live data, like iCarsoft, Autel or Snap-on) to read fault codes and check sensor values. Basic code readers from Halfords won’t show live data or freeze-frame info.
Step 1: Read the fault codes
Plug in your scanner, turn the ignition on (engine off), and read the stored fault codes. Write down all codes and their descriptions. If you see multiple codes, note which one appeared first (check the freeze-frame data or “first occurrence” timestamp).
Step 2: Check live sensor data
Go to the live data screen and check the following values with the engine running:
- AdBlue level: Should match the actual tank level (e.g. if tank is half-full, sensor should read 50%). If it reads 0% when the tank is full, the level sensor has failed.
- AdBlue pressure: Should be 5–9 bar when the engine is running and the pump is active. If it reads 0 bar or fluctuates wildly, the pressure sensor or pump has failed.
- AdBlue temperature: Should match ambient temperature when the engine is cold (e.g. 5–10°C on a winter morning). If it reads -40°C or stays at 20°C regardless of weather, the temperature sensor has failed.
Step 3: Inspect the sensor and wiring
If the live data looks wrong, inspect the sensor connector and wiring for:
- White crystals (dried AdBlue) on the connector pins
- Corrosion or green/blue deposits on the terminals
- Broken or chafed wires near the sensor
- Loose or disconnected plugs
Level sensor location: Inside the AdBlue tank, accessed via a hatch under the van or in the load bay (Sprinters, Crafters, Transits). You’ll need to drop the tank or remove the access panel to inspect it.
Pressure sensor location: On the AdBlue pump housing or in the delivery line near the dosing injector (usually under the van, near the DPF).
Temperature sensor location: Integrated into the level sensor (inside the tank) or on the pump housing.
Step 4: Clean or replace the sensor
If the sensor is covered in white crystals (dried AdBlue), try cleaning it first before replacing:
- Remove the sensor from the tank or pump (you may need to drain some AdBlue first)
- Rinse the sensor probe in warm water (not hot – thermal shock can crack the plastic housing)
- Use a soft brush (old toothbrush) to remove crystals from the probe and connector
- Dry thoroughly with compressed air or kitchen roll
- Refit the sensor, clear the fault codes, and test
If cleaning doesn’t fix it, or if the sensor is cracked or corroded, you’ll need a replacement. Genuine OEM sensors (Bosch, Continental, Delphi) cost £80–£200 depending on the vehicle. Aftermarket sensors are cheaper (£40–£100) but often fail within 6–12 months.
When to call a mobile AdBlue specialist
Some AdBlue sensor faults are DIY-friendly (cleaning connectors, topping up the tank), but others need specialist tools and experience:
- Level sensor replacement: Requires draining the AdBlue tank, dropping the tank or removing access panels, and recalibrating the new sensor using dealer-level diagnostics.
- Pressure sensor replacement: Requires bleeding the AdBlue system to remove air (similar to bleeding brake lines). If you don’t do this correctly, you’ll get P20E8 faults even with a new sensor.
- Temperature sensor / heater faults: Often requires replacing the entire pump assembly (sensor is integrated). Cost: £300–£600 for parts + labour.
- Wiring faults: Tracing broken or corroded wires in the AdBlue loom requires a multimeter and wiring diagrams. Bodged repairs often cause intermittent faults.
Our mobile AdBlue repair service in Stoke-on-Trent covers sensor diagnostics, replacement and system bleeding on-site at your home, workplace or roadside. We carry genuine and OEM-equivalent sensors for Sprinters, Transits, Crafters, Boxers and most Euro 6 vans, and we can reset fault codes and recalibrate sensors using Autotuner and dealer-level tools.
“Topped up my Sprinter in a Tesco car park in Hanley, and the warning light came on before I’d even left the bay,” Dave the courier told me. “Dash said ‘AdBlue empty – 300 miles to engine shutdown.’ I’d just put in 10 litres .” Turned out the level sensor was caked in white crystals and reading zero even with a full tank. We pulled the sensor, cleaned it with warm water and a toothbrush, refitted it and cleared the codes. Light went off instantly. Total time: 25 minutes. Saved him a £450 dealer bill for a new sensor that didn’t need replacing.
How to reset AdBlue sensor faults (after repair)
Once you’ve cleaned or replaced the sensor, you need to clear the fault codes and reset the AdBlue system. Simply disconnecting the battery won’t work on most modern vans—the ECU stores “permanent” codes that survive a battery disconnect.
Method 1: Using a diagnostic scanner
- Plug in your OBD2 scanner and turn the ignition on (engine off)
- Go to the fault code menu and select “Clear all codes” or “Erase DTCs”
- Turn the ignition off, wait 10 seconds, then restart the engine
- Let the engine idle for 30 seconds while the ECU runs a system check
- Check the dash—if the sensor is working, the warning light should stay off and the AdBlue range should update
Method 2: Dealer-level reset (for persistent faults)
If the warning returns after clearing codes, or if the dash still shows “AdBlue empty” even with a new sensor, you may need a forced regeneration and sensor recalibration using dealer tools (Mercedes XENTRY, Ford IDS, VW ODIS, etc.). This tells the ECU to re-learn the sensor values and reset the countdown timer.
We carry Autotuner and cloned dealer tools on our mobile van, so we can perform forced resets on-site without needing to take your van to a main dealer. Call 07503 134 362 if you need a same-day reset in Stoke-on-Trent or surrounding areas.
How to prevent AdBlue sensor faults
Most AdBlue sensor faults are caused by contamination, crystallisation or running the tank too low. Follow these tips to extend sensor life:
- Keep the tank above quarter-full – running low exposes the level sensor to air, which accelerates crystal buildup and corrosion.
- Use only sealed, ISO 22241 AdBlue – contaminated or watered-down fluid leaves deposits on sensors and triggers P20EE faults.
- Wipe the filler neck after topping up – spilled AdBlue drips into the tank breather and contaminates the level sensor.
- Don’t ignore low-level warnings – if you run the tank completely dry, air gets into the pump and pressure sensor, causing P20E8 faults that require bleeding to fix.
- Check connectors annually – if you can access the sensor connectors (some vans have inspection hatches), spray them with electrical contact cleaner once a year to prevent corrosion.
AdBlue Sensor Fault FAQ
How do I know if my AdBlue sensor is faulty?
Common symptoms include: dash showing “AdBlue empty” when the tank is full, warning light appearing immediately after topping up, “AdBlue pressure too low” messages, or the AdBlue range stuck at the same mileage even after refilling. Use a diagnostic scanner to check live sensor data—if the level reads 0% with a full tank, or pressure reads 0 bar with the engine running, the sensor has failed.
Can I clean an AdBlue sensor instead of replacing it?
Yes, in many cases. If the sensor is covered in white crystals (dried AdBlue), remove it and rinse in warm water, then scrub gently with a soft brush. Dry thoroughly and refit. This fixes about 60–70% of “faulty” sensors. If cleaning doesn’t work, or if the sensor housing is cracked or corroded, you’ll need a replacement.
How much does an AdBlue sensor replacement cost?
Level sensor: £80–£200 for the part, plus £50–£150 labour (1–2 hours). Some vans require dropping the AdBlue tank, which adds time and cost.
Pressure sensor: £60–£150 for the part, plus £40–£100 labour. Often requires bleeding the system after replacement.
Temperature sensor: Usually integrated into the pump or level sensor, so replacement cost is £100–£300 depending on the vehicle.
Will disconnecting the battery reset an AdBlue sensor fault?
No—not on most modern vans (Euro 6 and newer). The ECU stores “permanent” fault codes that survive a battery disconnect. You need a diagnostic scanner to clear the codes, and in some cases a dealer-level tool to reset the countdown timer and recalibrate the sensor.
Why does my AdBlue sensor keep failing?
Repeated sensor failures are usually caused by: contaminated AdBlue (water, dirt or wrong concentration), running the tank too low (exposes sensor to air and accelerates corrosion), using cheap aftermarket sensors (often fail within 6–12 months), or a wiring fault (corroded connector or chafed wire causing intermittent signal). If you’ve replaced the sensor twice in 12 months, get the AdBlue quality tested and check the wiring.
Can I drive with an AdBlue sensor fault?
Yes, but only until the countdown reaches zero. Once the ECU detects a sensor fault, it assumes the AdBlue system isn’t working and starts a countdown (usually 500–1,200 miles depending on the vehicle). When the countdown hits zero, the engine won’t restart. Get the fault diagnosed and fixed before you run out of miles.
What’s the difference between P20EE and P204F fault codes?
P20EE = “SCR NOx catalyst efficiency below threshold” – the ECU thinks the AdBlue system isn’t reducing NOx emissions enough. Usually caused by low AdBlue level, contaminated fluid, or a failed level sensor.
P204F = “Reductant system performance” – broader fault indicating a problem with AdBlue delivery (could be level sensor, pump, injector or contaminated fluid). Often appears alongside P20EE.
Where is the AdBlue level sensor located?
Inside the AdBlue tank, accessed via a hatch under the van (Sprinters, Crafters) or in the load bay (Transits, Boxers). On some vehicles you need to drop the tank to access the sensor. The sensor has a long probe that extends to the bottom of the tank to measure fluid level.
For related AdBlue fault guides, see our AdBlue pump vs heater fault comparison, Mercedes Sprinter fault code guide (P20E8 to P204F), or top AdBlue fault codes page.
AdBlue sensor fault won’t clear?
We diagnose, clean and replace AdBlue level, pressure and temperature sensors on-site across Stoke-on-Trent, Newcastle-under-Lyme, Stafford and Crewe. Same-day service available.
07503 134 362 |
✉ info@adbluespecialist.co.uk
Hours: Mon–Sun 09:00–20:00
