What are the causes of excessive nitrogen oxides in vehicle exhaust?

[VonFrancisco]

Common reasons for excessive nitrogen oxides in vehicle exhaust include overly rich air-fuel mixture, engine oil burning, and failure of the three-way catalytic converter. Other factors may include obstructed intake systems, engine carbon buildup, cylinder wear, and malfunctioning oxygen sensors. If CO and HC levels are within limits or slightly exceeded while only NOX is significantly high, it can be preliminarily determined that the three-way catalytic converter is faulty. Possible causes include catalytic converter poisoning or aging. Adding two bottles of exhaust cleaner to wash the converter or removing and cleaning the three-way catalytic converter on the exhaust pipe can resolve the issue. Overly rich air-fuel mixture: The main causes for this condition are problems with the engine coolant temperature sensor and oxygen sensor, which lead to inaccurate fuel supply calculations by the engine control unit. If excessive fuel isn't completely burned, it results in an overly rich mixture. Engine oil burning: This also affects exhaust emission standards because oil consumption indicates poor engine condition, leading to incomplete combustion. This increases nitrogen oxides, carbon monoxide, and hydrocarbon levels in the exhaust, causing emission standard violations. Three-way catalytic converter failure: After prolonged use, the converter may lose effectiveness, reducing its efficiency in treating exhaust gases. This is one of the reasons for failing emission standards.

[DeRyan]

Once during my car's annual inspection, it was found that the nitrogen oxide emissions exceeded the standard, which was really troublesome. Later, after consulting someone, the main reason was identified as the engine combustion chamber temperature being too high, especially during long-distance high-speed driving, where the mixture of air and fuel generates high temperatures, forming nitrogen oxides. Another key point was the aging of the three-way catalytic converter. Neglecting regular maintenance led to carbon buildup or damage, rendering it ineffective. Originally, this component could convert harmful gases into clean emissions. EGR system clogging is also quite common, preventing the exhaust gas recirculation from functioning properly and thus failing to reduce the temperature. Additionally, a dirty air filter affects the air intake volume, and poor fuel quality exacerbates incomplete combustion. I realized that exceeding the standards not only risks failing the annual inspection but also pollutes the air and increases the risk of acid rain. So now, I regularly clean the system, use high-quality fuel, and avoid letting small issues escalate into major problems. Remember to check component functionality in time and not wait until something breaks to fix it.

[JakeLynn]

In car enthusiast circles, we've discussed exhaust emission issues extensively, with nitrogen oxide (NOx) exceeding standards due to various reasons. The most common culprit is a clogged or stuck EGR valve caused by carbon buildup, preventing exhaust gas recirculation for temperature reduction and leading to skyrocketing combustion temperatures. Faulty oxygen sensors are another minefield—they regulate the air-fuel mixture ratio, and once inaccurate, the disrupted ratio causes emissions to spike dramatically. A failing three-way catalytic converter, whether due to high-temperature melting or poisoning, significantly reduces efficiency. Excessive engine carbon deposits also worsen combustion. My advice: don't overlook minor faults; conduct regular emission tests and prioritize checking these points when levels exceed standards. Quality fuel and proper maintenance are key—don't penny-pinch only to incur greater losses. Keeping the system clean extends its lifespan and makes a notable environmental contribution.

[MacEliza]

I've encountered NOx over-limit issues with several cars I've driven. It's mainly caused by high combustion temperatures, such as nitrogen oxides at high RPMs. The most common problem is EGR system failure, where the valve gets clogged and stops functioning. A damaged catalytic converter can't transform the gases. Poor fuel quality leads to incomplete combustion. Sensor issues cause control failures. Exceeding limits affects passing emissions tests and harms health. Focus on repairing these during inspections, and address them promptly.