Possible causes

– Faulty front heated oxygen sensor
– Ignition misfiring
– Faulty fuel injectors
– Exhaust gas leaks
– Incorrect fuel pressure
– Faulty Mass Air Flow (MAF) sensor

When is the code detected?

– Fuel injection system does not operate properly.
– The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Excessive Fuel Consumption

P0172 HONDA Description

With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios.
In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the MIL (2 trip detection logic).

HONDA Technical Service Bulletins for P0172 code