Engine control module (ECM)

The system controlling the automobile engine is called Engine Control Module(ECM) or Powertrain Control Module (PCM). The ECM is programmed to ensure the engine is running in optimal condition. One of the key missions of ECM is the perfect combustion with the right mixture of fuel and oxygen by completely burning fuel and leaving no left over fuel or air.

Stoichiometric index

This perfect ratio is called stoichiometric index. For gasoline engine,  the stoichiometric air–fuel mixture is about 14.7:1.  What this means is that 14.7 grams of air for one gram of fuel is the perfect ratio. ECM controls the injector opening time to maintain the 14.7:1 ratio.

System lean or rich

For 4 cylinder gasoline engine. If more air is flowing into the engine, then ECM injects more fuel to match the 14.7:1 ratio. This is what we are calling the engine is running lean possibly producing - P0171 DTC. On the other hand, if too little air is coming into the engine then injects the less fuel. We say the engine is running rich with possible DTC P0172.

Possible causes

1. Dirty Mass Air Flow (MAF) sensor.
2. Vaccum leak after the MAF sensor.
3. PCV valve stuck,loose connection, leak, hose leak.
4. Dirty fuel filter, fuel pump malfunction, fuel line clogged.
5. Injector malfunction or leak, low fuel pressure.

Possible solutions

1. Clean the mass air flow sensor.
2. Inspect vaccum leak and fix/replace PCV valve, hose, and connectors.
3. Test fuel pressure, replace pressure regulator, fuel filtre and/or fuel pump.
4. Test and replace the oxygen sensors.
5. Test injector leak, replace injector hose, connectors and injectors.

Further readings

1. Learn the basics of fuel trims to better understand the issues.
2. Learn short term fuel trim (STFT) and long term fuel trim (LTFT).
3. Also learn P0172 system too rich condition.

Notes. Above explanation is for gasoline internal combustion engine. Diesel and tunned engins may not be applicable to above.