What Causes a Vehicle to Fail Emission Standards?
3 Answers
High sulfur and phosphorus content in gasoline can form chemical complexes on the surface of the oxygen sensor and the three-way catalytic converter; congested urban roads, frequent vehicle starts and stops, and idling lead to incomplete combustion, with byproducts adhering to the surface of the three-way catalytic converter; engine carbon deposits and particulate matter can clog the three-way catalytic converter, which are the primary reasons for a vehicle failing emission standards. Below are solutions to address emission failures: 1. Shift to 4th gear and drive at 5th gear speed with wide-open throttle for several dozen kilometers. Principle: Congested urban roads with frequent stops and starts result in incomplete combustion, producing pollutants that adhere to the three-way catalytic converter. At high RPMs, gasoline combustion emits the lowest levels of pollutants, and high-speed airflow can also flush away deposits on the three-way catalytic converter. 2. Keep the engine warm and idling without shutting it off. Before inspection, rev the engine in neutral for 3-5 minutes. Principle: Testing a cold vehicle is unwise. The reason is simple—the optimal operating temperature for the three-way catalytic converter is between 400 to 800 degrees Celsius, and it takes a long time for a cold engine to reach 400 degrees. Revving the engine for a few minutes before testing can rapidly raise the three-way catalytic converter's temperature above 400 degrees and maintain it for a period. 3. Regularly clean the three-way catalytic converter. Principle: Chinese gasoline has high sulfur and phosphorus content, forming chemical complexes on the oxygen sensor and three-way catalytic converter surfaces. Carbon buildup in the fuel system and intake tract, as well as particulate matter, can contaminate the three-way catalytic converter. This leads to restricted exhaust flow, increased backpressure, higher fuel consumption, reduced power, and diminished or even failed purification function of the three-way catalytic converter. In severe cases, it can clog the converter, potentially causing vehicle self-ignition.
My old car failed last year’s inspection because of the exhaust. When we took it apart, the catalytic converter was completely clogged. The mechanic said this often happens from too much city driving—short trips lead to incomplete combustion, and carbon deposits get stuck in the honeycomb pores of the catalytic converter. There was also a loose oxygen sensor connector, which is responsible for adjusting the air-fuel ratio. If the connection is poor, the ECU starts making random adjustments. You really need to pay attention to oil quality—I replaced the OEM filter and cleaned the throttle body while I was at it, and the next inspection passed in one go. Engine carbon buildup shouldn’t be underestimated either, especially for those often stuck in traffic. A deep carbon cleaning every 20,000 km is a must.
Excessive exhaust emissions are mostly caused by incomplete engine combustion. Last time I encountered this issue, it took me a long time to figure out that it was due to aging ignition coils causing weak ignition in individual cylinders. Modern cars are so precise that even a slight deviation in the air-fuel ratio can cause emissions to exceed standards. Air leaks in the intake system can also cause problems; if the manifold gasket is aging and leaking, the ECU will mess up the mixture ratio. It's recommended to check the condition of the spark plugs and replace rubber hoses after about eight years of use. If you smell gasoline, be extra cautious—it's likely due to failed fuel line seals. Don't believe fuel additives can solve the problem; the key lies in basic maintenance.
