Under the National VI standard, ultra-low emissions are achieved through source control and post-treatment upgrades. For diesel vehicles, the architecture consists of DOC (Diesel Oxidation Catalyst) + DPF (Diesel Particulate Filter) + SCR (Selective Catalytic Reduction) + ASC (Advanced Scrubbing System) in four stages. During maintenance, the process must strictly follow the sequence of electricity before gas, and soft components before hard ones. The key points to focus on include the regeneration logic, sensors, and fuel quality.
I. Key Points of the National VI Standard and Technological Upgrades.
1. Core limit values: NOₓ reduced to 35 mg/km (a decrease of approximately 42%), PM3 mg/km. New control on particle number (PN), introduction of RDE actual road testing.
2. Mainstream solution for diesel vehicles: DOC + DPF + SCR + ASC four-stage series connection. DOC oxidizes CO/HC and supplies NO₂ to DPF, SCR reduces NOₓ with urea, ASC prevents ammonia leakage.
II. Key Components and Maintenance Points.
GPF/DPF (Particulate Filter)
Principle: Ceramic carrier with wall-flow design, with a filtration efficiency of over 90%, and carbon smoke is removed through regeneration.
Regeneration methods:
Passive regeneration: DOC produces NO₂ to assist combustion, with an exhaust temperature of 300–450℃.
Active regeneration: The ECU injects fuel at a temperature of 550℃ or higher, including driving and idling regeneration.
Faults and treatments:
Blockage: Pressure difference > 15 kPa, torque limitation, increased fuel consumption; first drive at 80–100 km/h for 20–30 minutes to promote regeneration, if ineffective, professional disassembly and cleaning or replacement are required.
Causes: Long-distance short trips, low ash content engine oil (API SP/ACEA C5), poor quality fuel.
2. DOC (Oxidative Catalysis)
Principle: Pt/Pd/Rh precious metal catalyst. Requires theoretical air-fuel ratio of 14.7:1 and operating conditions at 700°C +.
Faults and Troubleshooting:
Poisoning / Deactivation： Sulfur, lead, and silicon coating the surface, resulting in excessive CO/HC emissions and low efficiency (report P0420).
Treatment: Burn in a high-temperature furnace or replace the catalyst. Preferably use compliant fuel and low aircraft oil.
3. SCR System
Principle: 32.5% urea decomposes to produce NH₃, and reduces NOₓ to N₂ and H₂O.
Faults and Troubleshooting:
Crystallization/Blockage: Crystallization in urea nozzles and pipelines, with NOₓ exceeding the limit; soak and clean with 80°C hot water + special cleaning agent; replace the pump/nozzle if necessary.
Key: Use urea in accordance with ISO 2241 standard, with sulfur content ≤ 10 ppm in fuel.
4. Core Sensor (Critical for Accurate Diagnosis)
Pressure differential sensor: Measures the pressure difference before and after the DPF, causing failure of regeneration or torque limitation.
Exhaust temperature sensor: Monitors the temperatures of DOC/DPF/SCR, determining the regeneration strategy.
NOₓ sensor: Feedbacks the concentration of nitrogen oxides, controlling the amount of urea injection.
Oxygen sensor: Uses a dual-sensor closed-loop control of the air-fuel ratio, aging causing increased fuel consumption and low efficiency.
Maintenance: Reads OBD fault codes (P244B, P20EE, etc.), checks circuits/connections, and performs calibration.
III. Maintenance Practical Operation Procedure.
1. Read the code and observe the flow: Use the diagnostic instrument to check the DPF pressure difference, exhaust temperature, NOₓ values before and after regeneration, and the regeneration status.
2. First check for soft faults: Oil quality, engine oil, air filter, EGR carbon deposits.
3. Regeneration treatment: Perform driving/regenerative operation or stationary regeneration according to the scenario. If ineffective, disassemble and clean.
4. Urea system: Check the liquid level, pump, nozzle crystallization, and pipeline leakage.
5. Sensors and wiring: Measure power supply/signaling, CAN communication. Prefer original factory parts.
6. Post-treatment body: Clean or replace sintered/cracked parts.
IV. Key Points for Daily Maintenance and Care.
Oil: Must use API SP/ACEA C5 low ash (ash ≤ 0.8%), to avoid GPF ash accumulation.
Fuel: Regular Euro VI fuel, sulfur ≤ 10 ppm, refuse private fuel.
Regeneration habit: 80 km/h speed + constant speed for 20–30 minutes per week, avoid long-term short trips.
Maintenance cycle: Check GPF pressure difference every 10,000 km/6 months; Clean urea system every 30,000 km.
Prohibition: Strictly prohibit illegal shielding/ dismantling of after-treatment systems, which will affect compliance and pose high risks.
V. Quick Reference for Common Faults
Torque limit / power reduction: DPF blockage, sensor failure, abnormal urea system → First read codes, check regeneration and oil quality.
Increased fuel consumption: Nitrogen oxide sensor failure, air-fuel ratio imbalance, GPF blockage → Measure nitrogen oxide sensor, check pressure difference.
Strong exhaust odor: DOC deactivated, insufficient regeneration → Measure temperature, efficiency and regeneration status.