Do-it-yourself auto repair

P0172: System Too Rich Error. р0172 System Too Rich Diagnostic Trouble Code

P0172 Error means System Too Rich. Thus, combustion cylinders are supplied with too rich fuel mixture. As the code P0171, a rich mixture error is a system one. That is, it does not indicate a clear failure of sensors, but the parameters of the fuel amount are beyond the limit.

Diagnostic Trouble Code (DTC) P0172 OBD II

Depending on the reason that caused the appearance of the error code, the car state is different as well. In some cases, the fuel consumption will be conspicuous, some flooding at idle or speed drift, of both hot and cold engine.

Error Signal Conditions

The engine must run and the fuel supply takes place with feedback from the oxygen sensor (lambda probe), with no errors from the coolant sensor, intake air temperature sensor, absolute pressure (MAP sensor), camshaft and crankshaft position sensors, and the throttle position sensor, when the average total value of short- and long-term adjustment of the fuel is less than 33% in just over 3 minutes of the 7 trial period. Warning light on dashboard goes out only if diagnosis does not determine failure during the three test cycles.

Possible p0172 Error Causes

To understand what caused the system too rich error, it is necessary to make a list of reasons using a small algorithm.

Mixture enrichment arises from the incomplete combustion (excessive supply or air disadvantage); when fuel is not burned, it means that spark plugs or coils work poorly; when it is supplied with an excess, there is the oxygen sensor or nozzle fault; if there is no air, air consumption sensor gives incorrect data.

Fuel excess can rarely happen, but the lack of air is a typical problem. The air supply to the fuel occurs in interconnection of MAP sensor and lambda probe. But apart from the sensor, the problem can also be caused by a violation of the thermal gap (autogas system engines), mechanical damage to the various gaskets and seals, timing belt disruptions, or lack of compression.

To deal with all possible sources entailing failure, check is carried out in following steps:

  1. Scanner Information Analysis;
  2. Imitation before particular failure occurrence;
  3. Check of systems and components (good contact, no choke, good performance), which may result in p0172 error.


On the basis of above data, it is possible to identify the main causes as follows:

  1. Mass airflow sensor, its pollution, damage, loss of contact.
  2. Air filter clogging or air leak.
  3. Oxygen sensor, its malfunction (degradation, damaged wiring).
  4. Absorber valve, its malfunction affects the petrol vapor.
  5. Fuel Rail pressure. Over pressure can be caused by a faulty pressure regulator, damaged fuel system.


System Too Rich Error Remedy

Therefore, to find the guilty nozzle or system, you need to check MAF, engine temperature sensor and lambda probe with a multimeter. Then, check out the plugs, high-voltage wires and coils. Measure the pressure with a manometer. Check the label of ignition. Also check the connections on the air inlet and exhaust manifold for the presence of air leaks.

After correcting the problem, you will need to reset the fuel supply adjustment, in order to reset the long-term adjustment to 0%.

Having implemented all the recommendations, you will probably be able to cope with incorrect operation of the engine and P0172 trouble code diagnosis as for VAZ vehicles, foreign cars like Toyota or Mercedes, as well as other cars with electronic control. Although, all items are not often to be performed, in most cases, washing or replacement of mass airflow sensor or the oxygen sensor is enough.