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PEX Aftermarket Nitrogen Oxide (NOx)

Purpose and Function:

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NOx sensors in heavy-duty diesel engines measure nitrogen oxide levels in the exhaust gas. They provide real-time feedback to the ECU and SCR system to:

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  • Optimize urea/DEF dosing.

  • Control exhaust gas recirculation (EGR) and combustion parameters.

  • Ensure compliance with EPA Tier 4, Euro VI, and other emission standards.

  • Two-channel sensors (upstream/downstream) – monitor both raw exhaust and post-catalyst NOx levels to ensure SCR efficiency.

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Features & benefits

  • Accurate NOx measurement across high-temperature, high-flow diesel exhaust streams.

  • Enables precise SCR control, reducing urea usage and lowering operating costs.

  • Helps prevent emissions violations and avoids engine derates triggered by faulty readings.

  • Supports diagnostics of exhaust aftertreatment components like SCR catalyst, DPF, and EGR.

  • Rugged construction designed for harsh environments: extreme heat, vibration, and corrosive gases

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Technical Highlights

  • Signal output: Typically a current or voltage signal proportional to NOx concentration; some heavy-duty sensors include built-in heaters for rapid warm-up.

  • Placement:

  • Upstream sensor: monitors raw exhaust to optimize SCR dosing.

  • Downstream sensor: verifies NOx reduction efficiency across the SCR catalyst.

  • Operating temperature: Can tolerate exhaust temperatures up to 800–900°C, depending on engine and sensor design.

  • Response time: Fast response (<1 second) critical for controlling DEF injection under varying loads.

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Diagnostic and Testing Tips

  • Visual inspection: Check for soot, oil, coolant contamination, damaged wiring, or corroded connectors.

  • Electrical test: Verify proper heater resistance, supply voltage, and sensor output per OEM specs.

  • Scan tool check: Read live NOx readings upstream and downstream; ensure values react to engine load changes.

  • Functional comparison: Confirm downstream NOx readings are significantly lower than upstream, indicating SCR efficiency.

  • Replace sensors that fail to respond, show inconsistent output, or have heater faults. Always inspect SCR, DPF, and EGR systems for proper operation when faults occur.

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Installation Tips and Common Issues

  • Route sensor harnesses away from hot exhaust, moving parts, and sharp edges; use heat shields if necessary.

  • Torque sensor threads per manufacturer spec to prevent leaks; use thread sealant if recommended.

  • Avoid contamination from silicone, oil, or coolant, which can permanently damage the sensor.

  • Sensor life is affected by excessive soot accumulation or DEF crystallization; periodic inspection and cleaning of SCR components can extend service life.

  • Intermittent faults are often caused by wiring damage or connector corrosion, not the sensor itself.

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