A car running rich (too much fuel) usually smells of gasoline, fails emissions, and gets bad mileage. The cause is almost always a faulty O2 sensor, leaking injector, or bad coolant temp sensor.
A dead or slow upstream sensor stops the ECU from trimming fuel. The default is rich. Codes P0131-P0167 identify the bad sensor.
A stuck-open or dribbling injector dumps extra fuel into one cylinder. Often shows a misfire code on that cylinder.
A sensor stuck reading cold makes the ECU keep enriching for warm-up. Often paired with code P0117 or P0118.
A regulator that leaks vacuum or sticks high adds fuel pressure beyond spec. Pull the vacuum hose - if you see fuel, replace it.
A contaminated MAF reads more airflow than reality. The ECU adds fuel to match imagined air. Quick clean usually fixes it.
You smell fuel inside the cabin, see fuel dripping from the engine bay, or the catalytic converter glows red. Severe rich running can ignite excess fuel in the exhaust.
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The air-fuel ratio is below 14.7:1 - too much fuel for the air. You lose power, foul plugs, and damage the catalytic converter.
Yes, eventually. Excess fuel burns inside the cat and melts the ceramic substrate. A $1,000+ repair.
Mildly, by reducing airflow. But fuel trim usually compensates. A truly clogged filter is more likely to cause hesitation than rich codes.
Rich smells like gas and shows fuel trim negative. Lean shows fuel trim positive and may misfire. P0172 = rich, P0171 = lean.
No. Rich means the ECU is adding too much fuel, not that the fuel itself is wrong. Fix the sensor or injector instead.