An SCR system fault means your diesel’s emissions system has failed, and ignoring it can cost you thousands. Read on to understand the working of a diesel emission system and how you can get it fixed in no time.
“Did you know? According to a report by Science Direct, the SCR system is employed to reduce the NOx emissions.”
Key takeaways
- An SCR system failure is quite a major issue, and you definitely shouldn’t ignore it. It can result in limp mode and costly repairs if left unaddressed.
- SCR faults are, first and foremost, caused by a low level or contaminated DEF fluid. So first, make sure that you check the fluid level and quality.
- Some 30% of fault codes are related to the NOx sensor issues. To avoid wrong readings, always purchase OEM replacement sensors.
- Without a diagnostic scan tool, a mechanic can only guess it’s the source of the fault, which might cause great expenses for the wrong parts.
- SCR failures can be avoided by regular maintenance. For example, a 25,000-mile interval DEF quality check and system cleaning can save from breakdowns and costly expenses.
SCR systems fail more commonly than any other powertrain-related system in diesel vehicles. If your dashboard has shown a warning, then you are definitely not the only one. At 30 Minute DPF Clean, this is one of the most common issues diesel owners call about every week.
What is a SCR system, and why does it matter?
So, what is a SCR system exactly? SCR is Selective Catalytic Reduction. It is a diesel emission control system installed on modern diesel vehicles. The main function of the SCR is to cut down the harmful nitrogen oxides or NOx gases emitted through your vehicle’s exhaust.
Here is how it works, step by step:
- Exhaust gases leave the engine
- In the exhaust flow, Diesel Exhaust Fluid (DEF) or AdBlue is injected into the exhaust stream.
- The fluid combines with the heated gases, and because of this, the mixture goes through the SCR catalyst.
- The catalyst then changes NOx into harmless nitrogen and water vapor.
The major components of the system are the DEF tank, DEF pump, DEF injector (or doser), SCR catalyst, and NOx sensors. When any of these components fail, the system triggers a fault.
Warning signs: Recognising a diesel SCR system fault
Dashboard lights and power loss symptoms
Your vehicle will almost always give you early warning signs. Don’t overlook these signs. Some typical indications of a problem with a diesel SCR system are:
- A warning light on your dashboard (often amber or red)
- Reduced engine performance or activation of limp mode
- Increased fuel consumption
- More DEF consumption than usual
- Black or excessive exhaust smoke
- Rough engine running.
Symptoms may develop slowly or very suddenly. In extreme situations, the Engine Control Unit (ECU) may restrict your vehicle’s speed down to 5 mph only to protect the environment and the engine. This is called derate mode, and it will make your vehicle almost unable to drive.
Common causes of a SCR system fault
Low or poor-quality DEF fluid
This is the main reason. DEF has to keep the urea concentration at 32.5% exactly (with a permissible range of 31.8% to 33.2%). The system will set off a fault straight away if the fluid is running low, past its expiry date, or contaminated.
Clogged or faulty DEF injector
DEF liquid is susceptible to crystallisation within the injector if it is subjected to elevated temperatures or oxygen. The presence of white crystalline residues surrounding the injector tip definitely indicates such a situation. An obstructed injector acts as a barrier for the DEF to flow into the exhaust system.
Faulty NOx sensors
About 30% of all SCR-related fault codes are due to sensor malfunctions. NOx sensors are exposed to very harsh environments, such as high temperature levels and corrosive gases, etc. Gradually, carbon residues accumulate, causing the sensor readings to drift, which results in the dosing mechanism being wrongly operated or false warnings being given.
Failed DEF pump
The DEF pump delivers fluid to the injector. If it fails, DEF injection stops entirely. This results in an immediate fault and engine derate.
SCR catalyst degradation
Over time, the catalyst can get covered with soot and other harmful substances, which not only makes it less effective but also causes it to throw up fault codes related to its efficiency, like P20EE (catalyst efficiency below threshold) or P207F (fluid quality issue).
Electrical and wiring faults
Chafed wires, connectors which are corroded connectors, and communication hiccups between the Electronic Control Unit can all be causes of SCR faults, even if the mechanical parts have no problems. These are harder to diagnose and require a proper scan tool.
SCR fault code reference table
| Fault Code | Meaning | Likely Cause |
| P20EE | SCR catalyst efficiency below threshold | Worn catalyst or poor DEF |
| P207F | Reduced the quality performance issue | Contaminated or expired DEF |
| P204F | SCR system performance fault | Multiple component failure |
| P20EF | NOx pre-catalyst efficiency is low | DEF injection failure or sensor fault |
How to fix SCR system fault: Step-by-Step
If you are wondering how to fix the SCR system fault, here is a step-by-step process:
Step 1: Check DEF level and quality first
Start with the simplest check. Add more DEF to the tank if it is almost empty. In case you think the fluid is not only old but also contaminated, then it is better to get rid of it and refill the tank. Check with a refractometer to make sure the urea concentration is between 31.8% to 33.2%.
Step 2: Run a diagnostic scan
Never guess when you can read the data. Use an OBD2 or a professional diagnostic scanner to access the fault codes. Clean codes of SPN and FMI will pinpoint the exact component that is broken. It helps in saving expenses by not changing the parts that are not faulty.
Step 3: Inspect for visible issues
Look for:
- White crystal deposits around the DEF injector
- Cracked or leaking DEF lines and hoses
- Corroded wiring or loose electrical connectors
- Damage to the SCR catalyst housing
Step 4: Clean or replace the faulty component
Once the fault is identified:
- Blocked DEF injector: A thorough cleaning with a specific DEF system cleaner is usually enough. If the injector is soiled beyond cleaning, it should be replaced.
- Faulty NOx sensor: The best solution is to get a new sensor directly from the manufacturer. Sensors from other brands are known for creating bogus fault codes, and because of this should be avoided.
- Failed DEF pump: This can only be done by a replacement. Deferring this kind of repair is not an option.
- Degraded catalyst: In some cases, cleaning will bring back the performance to a good level. If the degradation is very bad, then the only option is complete replacement.
Step 5: Clear fault codes and test
Clear all stored fault codes after repairs using your diagnostic tool. Besides, run the engine and watch for any warning lights. If the fault comes back, it might be a hidden problem that needs a professional diesel emissions service.
Struggling to clean your DPF Filter? Get professional help today!
If the steps above have not resolved your SCR system fault, it is time to call the specialists. Thorough diesel emissions servicing entails more than just a simple reset. Here at 30 Minute DPF Clean, our experts rely on industrial-standard instruments to properly check, clean, and revive your diesel SCR system as if it were brand new. This cost saves you the cost of unnecessary part replacement. Don’t let a minor malfunction turn into a major expense. Book your services today.
FAQs
- Can I drive with an SCR system fault?
It is possible to drive for a short distance if necessary, but it is definitely not a good idea. The vehicle is designed to enter limp mode when this happens, so power will be greatly reduced. Also, by continuing to drive, you tend to cause additional damage to the emission system components, which in turn, will make the final repair as well as the costs of repair much higher.
- How much does it cost to fix an SCR system fault?
Costs can greatly differ based on the cause. Simply topping up DEF fluid might only cost you a few pounds. Sensor replacements generally fall between 100 and 400. A broken SCR catalyst might cause a bill of 800 to 5,000 or even more. First, getting the right diagnosis saves you from unnecessary spending.
- Will clearing the fault code fix my SCR system?
First of all, clearing a code is a means of removing a warning from the instrument panel for a while. Yet, the light and code will come back once the engine is checked again by the computer if the problem is not corrected. This way, resetting a code is only a diagnostic procedure and not a repair at all.
- What causes white crystals around the DEF injector?
White crystalline deposits form when DEF fluid is exposed to air or if it is subjected to elevated temperatures. Exposure of the urea contained in the fluid to the atmosphere leads to drying and solidification of the urea tissues, with the crystalline nature marking the scale. Given the fact, the buildup can be removed with a proper cleaning agent, or the injector itself can be replaced.
- How do I prevent the SCR system faults from happening?
Make sure you use only quality DEF fluid and keep the tank topped up. Check fluid concentration with a refractometer every 25,000 miles. Do not allow the DEF tank to run empty. Get the diesel emissions service performed at the manufacturer’s recommended intervals to maintain system cleanliness.
