Ammonia Smell From the Exhaust: SCR Causes & Diagnosis

What the Ammonia Smell Actually Is (and Why It Happens)

In an SCR system, DEF/AdBlue® (aqueous urea solution) is injected into the exhaust stream. It breaks down into ammonia (NH₃), which reacts with NOx in the SCR catalyst. When everything is working correctly, ammonia is consumed in the reaction and you should not notice any ammonia smell at the tailpipe.

An ammonia smell usually indicates ammonia slip—meaning excess NH₃ passes through the catalyst without reacting. This can happen when:

• Too much DEF is injected (over-dosing)
• NOx sensor feedback is incorrect (faulty NOx readings)
• The catalyst cannot convert efficiently (saturation or reduced activity)
• Exhaust temperature is outside the effective SCR operating window

Cause #1: SCR Over-Dosing (Too Much DEF/AdBlue®)

Over-dosing is one of the most common causes of ammonia smell. The ECU calculates DEF dosing based on NOx sensor values, exhaust mass flow, and temperature-based models. If dosing exceeds the catalyst’s conversion capacity, ammonia slip occurs.

Common Reasons for Over-Dosing

• Leaking DEF dosing unit: The injector drips instead of finely atomizing—especially after shutdown.
• Faulty dosing control: Issues in the pump/module can lead to excessive delivery.
• Software/calibration deviation: An incorrect calibration strategy (rare, but possible after updates or improper tuning).
• False “high NOx” value: The pre-SCR NOx sensor reports excessively high NOx; the ECU compensates with more dosing.

What Technicians Commonly Observe

• Ammonia smell especially after hard acceleration or long highway driving
• Crystal formation around the dosing inlet (white deposits)
• SCR-related fault codes that come and go depending on temperature
• Above-average DEF consumption

Cause #2: Incorrect NOx Sensor Feedback (the ECU Doses Incorrectly)

Euro 6/VI systems typically use two NOx sensors: one upstream of the SCR (pre-SCR) and one downstream of the SCR (post-SCR). The ECU uses these values for dosing control and to evaluate SCR efficiency.

If either NOx sensor is inaccurate, the ECU can dose incorrectly:

• Upstream NOx too high: ECU over-doses DEF → ammonia slip smell.
• Downstream NOx too high: ECU believes SCR efficiency is too low → increases dosing, even if the catalyst may be OK.
• Downstream NOx too low: Can mask a real SCR issue and delay diagnosis.

Causes of Incorrect NOx Feedback

• Aging/drift of the NOx sensing element
• Wiring/connector damage due to heat, corrosion, or poor ground
• Wrong sensor installed (not matched to the application)
• Exhaust leak upstream of the sensors, allowing oxygen ingress and skewing readings

Important: A NOx sensor can be “electrically OK” (no direct NOx sensor DTC) and still produce implausible values under real driving conditions. That’s why live data is critical.

Cause #3: Catalyst Saturation or Reduced Conversion Capacity

The SCR catalyst needs the right conditions to convert NOx efficiently. If the catalyst becomes saturated with ammonia or its conversion capacity declines, the likelihood of ammonia slip increases.

Typical Temperature/Catalyst-Related Scenarios

• Low exhaust temperature: SCR conversion is weak when exhaust is too cool; ammonia may pass through unreacted.
• Catalyst aging/poisoning: Thermal stress, oil ash, sulfur, or contaminants reduce activity.
• Aftertreatment imbalance: Combustion issues increase NOx/soot and stress the catalyst and dosing strategy.
• Ammonia storage overload: Repeated high dosing can “load” the catalyst with NH₃, especially in certain drive cycles.

Clues a Technician May Notice

• Ammonia smell strongest after long drives or under high load
• SCR efficiency fault codes even though NOx sensors appear normal
• Plausibility issues with temperature sensors or unusual EGT patterns
• Changes in DPF regeneration frequency that affect exhaust temperatures

Diagnostic Workflow for Technicians (Euro 6/VI – Practical Steps)

Use this step-by-step workflow to avoid guesswork and find the real cause of ammonia smell.

Step 1: Confirm Operating Conditions

• When does the smell occur? (cold start, highway load, after regeneration, after shutdown)
• Check DEF consumption trend (a sudden increase points to over-dosing)
• Verify the customer’s driving profile (short trips often keep SCR too cold)

Step 2: Read Fault Codes + Freeze Frame

• Document SCR/NOx-related codes and freeze-frame data
• Watch for linked temperature, dosing, and exhaust pressure codes
• Do not clear codes before saving the data

Step 3: Visual Inspection (Quick Wins)

• Check exhaust leaks upstream of the NOx sensors and around the SCR system
• Inspect the DEF injector area for white crystal deposits (leak/drip)
• Inspect wiring/connectors in hot zones (NOx and EGT sensors)

Step 4: Plausibility Check Using Live Data

• Compare upstream vs. downstream NOx values under steady load
• Confirm exhaust temperatures are in realistic operating ranges
• Observe commanded dosing/actuation (spikes or unusual patterns)

Rule of thumb: If upstream NOx is unusually high but engine behavior seems normal, check sensor plausibility first—before replacing a catalyst.

Step 5: Confirm NOx Sensor Health

• Check power/ground and connector integrity
• Watch for “flatlining” (value stuck) or unrealistic jumps during load changes
• If possible: compare with a known-good, correct OE reference sensor

Step 6: Check DEF Hardware

• Run dosing quantity tests (if available per OEM procedure)
• Inspect DEF injector spray pattern and check for dripping
• Test DEF quality/concentration if contamination is suspected

Step 7: Evaluate Catalyst Conversion Capacity

• If sensors and dosing are confirmed OK: evaluate SCR efficiency at correct temperatures
• If conversion remains low: consider catalyst aging/poisoning
• Check whether upstream combustion issues (injectors/EGR/boost) cause unstable NOx output

Technician tip: Many “SCR catalyst” replacements are actually caused by incorrect NOx feedback or dosing hardware issues. Always validate sensors first.

How DieselFixNeuss Helps with Ammonia Smell and SCR Complaints

DieselFixNeuss (Diesel Fix Neuss) supports workshops and fleets with application-specific NOx sensors and other diesel components that help stabilize aftertreatment performance. Because SCR dosing decisions rely heavily on correct NOx feedback, a drifting or mismatched sensor can cause over-dosing, ammonia slip, and recurring comebacks. View our products.

How we support sustainable repairs:

• Tested, OE-matched NOx sensors to restore correct pre-/post-SCR readings
• Fitment support by OE part number to avoid misapplication
• System-oriented support to link NOx, temperature, and dosing behavior to the true root cause

If you’re dealing with ammonia smell and recurring SCR faults, accurate NOx measurement is often the fastest path to a stable solution.

Conclusion

An ammonia smell from the exhaust is not “normal”—it usually indicates ammonia slip caused by SCR over-dosing, incorrect NOx sensor feedback, or catalyst saturation/reduced conversion capacity. The correct repair is based on structured diagnosis: confirm operating conditions, log live NOx and temperature data, check for leaks and crystal deposits, verify sensor plausibility, and only then evaluate catalyst performance.

With the right diagnostic workflow and OE-matched, tested NOx sensors from DieselFixNeuss, technicians can reduce unnecessary parts replacement, stabilize SCR function, and avoid expensive comebacks on Euro 6/VI diesel vehicles.