Mastering Diesel Functional Tests with Snap-on® Diagnostic Software

Learn how to get the most from Snap-on® Diagnostic Software when working on diesel vehicles. This article walks you through DPF regeneration, AdBlue dosing, and NOx sensor checks, while showing how to monitor the full diesel emission system workflow. For more detailed troubleshooting tips, a linked Technical Focus article provides extra guidance. 

 

Simplifying DPF, AdBlue, and Sensor Testing

Modern diesel vehicles feature complex emission control systems, from NOx sensors to SCR (Selective Catalytic Reduction) modules and DPFs (Diesel Particulate Filters). Professional diagnostics are critical for efficiency, compliance, and cost-effective repairs. 

 

Snap-on® Diagnostic Software, when kept up to date on the latest release, provides technicians with guided workflows and repair procedures, allowing them to: 

• Identify symptoms quickly 

• Get to the root cause efficiently 

• Verify repairs to prevent costly comebacks 

 

If you're not on the latest software release, technicians risk missing key updates, diagnostic functions, and guided procedures, which could slow down troubleshooting or lead to incomplete verification of repairs. 

For an in-depth look at diesel emission system verification, including NOx and SCR monitoring, see our Technical Focus Article, Understanding and Diagnosing Diesel Particulate Filter (DPF) Systems: Diesel System Diagnostics: NOx, SCR, and DEF Verification  

 

Understanding NOx and Ammonia Sensors 

NOx sensors are central to emission control, measuring nitrogen oxide levels at both the inlet and outlet of the SCR system. While highly effective, conventional NOx sensors can misidentify ammonia as NOx, leading to falsely elevated readings. Some manufacturers incorporate additional ammonia sensors to improve accuracy and prevent interference from other exhaust gases. 

Key Points: 

• Inlet and outlet NOx readings indicate SCR effectiveness. 

• Ammonia sensors serve as a backup to prevent false NOx readings. 

• Cross-checking scan tool data with a 5-gas or single-gas analyser ensures reliable measurements. 

Example Check: 

• Normal operation: 300 ppm in, 30 ppm out (≈90% reduction). 

• Total failure: 300 ppm in, 300 ppm out (sensor reading may be falsely high). 

Partial failure: Intermediate readings indicate malfunction in the NOx sensor or SCR system. 

 

Diesel Emission System Workflow 

DPF → Urea Injector → SCR → Slip Catalyst 

Typical diesel emission systems follow this sequence: 

 

Snap-on® Diagnostic Software allows technicians to monitor and test each stage: 

• DPF status and regeneration: Static or dynamic regen procedures, teach-in for replaced filters, and soot load verification. 

• SCR functional checks: Urea dosing, visual leak tests, injector and pump actuation. 

• Sensor verification: Confirm upstream/downstream NOx and ammonia sensors using scan data and optional tailpipe measurement.  

Safety Note: Manual DPF regeneration generates extremely high exhaust temperatures (>1000°F / 538°C). Always perform outside, away from personnel and combustible materials. 

Regen Types: 

• Passive: DPF temps high enough under load to burn soot naturally. 

• Active: Initiated by PCM or scan tool, extra fuel raises DPF temps to ~1025°F. 

• Parked: Dashboard-initiated when active regen fails. 

 

Diesel Functional Testing with Snap-on® Software 

The latest Snap-on® Diagnostic Software release gives technicians exclusive access to guided workflows that streamline diesel functional tests, helping to identify the root cause quickly and verify repairs with confidence. 

DPF Regeneration 

• Static and dynamic regeneration procedures. 

• Real-time soot load monitoring. 

• Safety prompts to protect the technician and the workshop. 

 

AdBlue® / SCR Testing 
SCR systems require precise dosing of AdBlue® to convert NOx into harmless nitrogen and water. Functional tests include: 

• Triggering dosing events to check spray patterns and flow. 

• Verifying tank level, heater operation, and pressure. 

• Cross-referencing NOx sensor feedback for accurate SCR efficiency checks. 

 

Sensor Integrity 
Reliable sensors are critical for emissions control. Snap-on® software enables technicians to: 

• Compare inlet vs outlet NOx readings. 

• Validate ammonia sensor accuracy. 

• Detect faulty readings that could mask system failures. 

 

Guided Component Tests 

Snap-on® exclusive guided component tests provide step-by-step instructions for sensors, pumps, heaters, and valves, allowing technicians to verify repairs and prevent costly comebacks. 

Example NOx Workflow: 

• Measure NOx at the inlet (e.g., 300 ppm). 

• Observe outlet NOx post-SCR (e.g., 30 ppm). 

• Cross-check with a single-gas or five-gas analyser if discrepancies appear. 

• Reset sensors or perform actuator tests as needed using Snap-on® guided component test procedures. 

 

Vehicle Specific Workflows 

While general coverage is essential, what matters most is how these tests apply to the vehicles you see in the workshop. Snap-on® diagnostic platforms provide a deeper functionality for a wide range of diesel vehicles, including advanced system support and embedded safety guidance. 

Snap-on® diagnostic tools provide full access to SCR and AdBlue/DEF systems. 

• Functional Tests: Reset AdBlue tank fill level, return tests, visual leak checks, and dynamic delivery pump operation. 

• Actuator Tests: Operate the AdBlue pump, set metering valves at 5% and 50%, and check tank and pressure line heating elements. 

• DPF Relearns: Supports Diesel Particulate Filter adaptation procedures for accurate system recalibration after maintenance or replacement 

This detailed coverage gives technicians the ability to confirm dosing, detect leaks, and validate system components, all from the same platform. 

Emissions functions are fully integrated into the engine menu for seamless access. 

• DPF Regeneration: Supports static (high RPM for extended periods) and dynamic (vehicle in motion) procedures; manual regen may take up to an hour. 

• Safety Prompts: On-screen guidance warns, for example, to perform regen outdoors due to extreme exhaust temperatures. 

• Additional Functions: Fuel Injection Correction Factors, Power Balance, SCR dosing measurement, refilling activations, and visual leak checks. 

• Exclusive Guided Component Tests: Step-by-step support with the latest Snap-on® software for DPF Differential Pressure Sensors and NOx Sensors, including reference waveforms for known-good and known-bad signals. 

The Jeep Renegade demonstrates how Snap-on® tools combine coverage with secure access. Through Snap-on® Security Link™, the scan tool automatically connects through the Secure Vehicle Gateway, enabling all required diagnostic requests. 

 

• Functional Tests: Turbocharger pressure actuator, swirl actuator commands, and DPF lamp activation. 

• System Tests: High and low pressure EGR valves, throttle valve, and electronic VGT actuator checks. 

• Miscellaneous Functions: NOx replacement, particulate filter regeneration, rail pressure sensor, injector, EGR valve, and turbo actuator replacements. 

• Emissions Coverage: Supports dosing control units, reductant components, NOx resets, and EGR/EGT checks. 

 

Guided Workflows for Confident Diesel Repairs 

For diagnostic technicians, mastering these diesel functional tests delivers tangible benefits: 

• Confident diagnostics across DPF, SCR and Nox systems. 

• Faster identification of symptoms and quicker access to the root cause. 

• Significant time savings through guided workflows and repair procedures. 

• Clear verification of system components and repairs using Snap-on® Exclusive Guided Component Tests 

 

Conclusion 

Snap-on® Diagnostic Software equips diesel technicians to master complex emission systems efficiently. From sensor verification to SCR and DPF functional tests, these tools streamline diagnostics, reduce errors, and maintain compliance. 

For more detailed guidance, including NOx and SCR verification techniques, see our full walkthrough in Diesel System Diagnostics: NOx, SCR, and DEF Verification.