Fault code P0172 – a step-by-step guide to diagnosing and fixing it

P0172 means that the air-fuel mixture is too rich for the car engine parts to run effectively, containing too much fuel or too little air. The ideal air-fuel ratio, known as the stoichiometric ratio, is about 14.7 If the ratio is too far out of the normal range, it can seriously affect the engine’s performance and the driveability of the vehicle.

• An excess of fuel due to a leaky fuel injector in a car, faulty pressure regulator, bad absorber valve, or a fuel pump that creates too much pressure.
• Air flow restrictions caused by a clogged car air filter, vacuum leaks, or problems with the lambda, MAF or throttle position sensor.
• Faults in the ignition system, such as worn spark plugs, bad ignition coils, broken high voltage wires, or timing errors.
• Cooling system problems. For example, a thermostat that won’t shut, a defective coolant temperature sensor, or a radiator fan that is constantly running. The uncontrolled circulation of coolant liquid may cause the system to use more fuel in order to heat the power unit.
• It is also possible that the engine control unit (ECU) is malfunctioning, meaning that the mixture, ignition and exhaust processes aren’t being regulated properly.

• A noticeable increase in fuel consumption
• A strong fuel or sulphur smell
• Black smoke coming from the exhaust system
• A decrease in engine power
• Misfiring and rough idling
• Sooty car spark plugs
• Jerking during acceleration
• Starting difficulties

• Connect an OBD2 scan tool to the vehicle to read the diagnostic trouble codes that have been registered by the system. Deciphering and linking the DTCs together will help you to identify the root of the problem quickly.
• Remove contaminants and rust from the electrical components and connector pins using a specialised cleaning agent. The sensors should be removed for inspection and cleaning.
• Check whether the electronic devices and cables are functioning properly using a multimeter and oscilloscope.
• Inspect the fuel lines for cracks, deformation and leaks. It is also a good idea to examine the vacuum and PCV hoses. Leaks can be fixed by installing new seals and replacing damaged or worn parts.
• Check the fuel pressure at the fuel rail with a pressure gauge
• Damaged, worn or defective parts that cannot be repaired using simple methods, such as a cleaning method, should be replaced. Once you have repaired the system, you can reset the DTCs and erase them from the computer’s archive using the scanner.