Air Fuel Ratio(AFR) incorrect leads to unstable idle speed even misfire!!

[jellyfish1234]

When I talking about misfire some days ago, got a comment saying that she had misfire before and it turned out to be sensor issue. Then it occurs to me that there is an occasion that sensors are working fine, the actual problem is AFR inappropriate. The syndrome of this problem could be car shaking/trembling, idle speed goes too high and too low, even misfire.

So I use my Topdon Artipad to scan the DTC, and got P0171. There are many possible reason for this code, you can google it, but I will summarize the essence is too much air too little fuel.

When AFR is incorrect, it actually means fuel is less and air too much, the essence is too much O2 in emission.

For fuel system, when air intake is fixed, the AFR will be incorrect if fuel intake is low due to component issue.
For exhaust system, when air intake is fixed, air leak, O2 sensor and AFR sensor detect and exhaust system related EGR should also be considered.
For the intake system, when air intake is fixed, sensor features change, leads to fuel intake less than normal.
For ignition system, when air intake is fixed, what’s the reason? Think about it, when misfire, mixed gas not completed burnt, O2 sensor or AFR sensor detect too much O2 in exhaust pipe.

Some inexperienced technicians might think O2 sensor and AFR sensor broken that cause this problem. But I think the exact opposite, the sensors are working just fine. P0171 and P0172 are generated because they are monitoring, but, if you don’t fix the issue asap, sensors will be dead after a while.

For my last article on misfire, you can click to my profile or search "my shallow analysis on engine misfire".