What causes excessive carbon monoxide in car exhaust?

2 Answers
McKimberly
07/24/25 12:53am
Excessive carbon monoxide levels are usually caused by a faulty catalytic converter leading to emission exceedance. Common reasons for high carbon monoxide include: 1. Carbon deposits: Caused by carbon buildup and coking (excessive throttle valve opening, oversized spark plug gaps, excessive intake system carbon deposits, etc.). 2. Oxygen sensor: If emission tests show exceedance, first check the oxygen sensor's working status with a diagnostic tool (oxygen sensors activate only when engine temperature exceeds 90°C). A faulty oxygen sensor may cause nitrogen oxide exceedance; oversized spark plug gaps lead to carbon monoxide exceedance; excessive carbon deposits result in hydrocarbon value exceedance.
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DiJeffrey
08/10/25 8:37pm
Last time when I sent my car for repair, it was also detected with excessive carbon monoxide in the exhaust. The mechanic used a diagnostic tool and found that the air-fuel mixture was too rich. Simply put, too much gasoline was being injected while too little air was entering, causing unburned fuel to turn into black smoke and white fog. Common causes might be a dirty air filter blocking the intake or an oxygen sensor covered in carbon deposits sending error signals. Another possibility is a leaking fuel injector dripping gasoline directly into the cylinder where it can't burn. The most troublesome issue is a failed catalytic converter, which is supposed to clean toxic exhaust gases but stops working completely once clogged. It's best to check these parts before the annual inspection to avoid making two trips to the testing station.
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