Diagnostic Trouble Code (DTC) P0172 Fuel Trim System Too Rich

Diagnostic Trouble Code (DTC) P0172

Fuel Trim System Too Rich



Features of the Circuit


  • There will be a deflection in the adaptive terms which is capable of identifying a slow default emerging in case the adaptive value threshold is permanently exceeded.
  •  As the lambda controller supersedes the adaptation threshold, the two time counters (one for the rich side and another one for the lean side) will be kicked started.
  • The error or the discrepancy will be detected the moment either of the time counters attains the highest value.
  • This test is undertaken keeping in mind that a failure that can exceed the adaptive terms can be generated.
  • The following two failures need to be created.

                            a)     A lean side deviation: P0171

                             b)     A rich side deviation : P0172

  • Determining the limit good and the limit bad for each kind of failure therefore becomes mandatory.
  • For any given failure, it is advised to measure the emission threshold until the legal emission thresholds are exceeded.
  • It must also be noted that the problem is because of the emission thresholds required, which is pretty complicated and the system needs to be affected by some disturbance so that the emission thresholds may be exceeded.
  • A judicious calibration has already made the tuning but, since the regulation does not approve of such a move it is mandatory to forcibly generate some visible material malfunction like (fuel pressure regulator, fuel injector, air leakage...)etc.




When will DTC set?

Task Boosted When DTC sets

When Does DTC clear?

1) When the system is in closed loop.

1) The Malfunction Indicator Lamp (MIL) will be seen glowing after three failed repetitive trips.


1)   The MIL will stop work after four repetitive ignition cycles where the diagnostic functions flawlessly.


2) When Manifold Absolute Pressure (MAP) ranges between 70 kPa (10.2 psi).


2) The ECM is devised to record operating conditions at the time the diagnostic fails which will be stored in the Freeze Frame and Failure Records will be boosted.


2)  A history DTC will clear after 40 consecutive warm-up cycles without a fault

3) When Coolant temperature > than 80°C(176°F).


3) A history DTC is stored.


3)  The Scan Tool can be made use of in order to clear all DTCs

4)  When  DTCs P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0337, P0338, P0341, P0342, P0400, P1319, P1402, P1404, P1405, P1671 and P1672 are not set.



4) Disconnecting the ECM battery feed for more than 10 seconds



                     Detailed Diagnostic Assistance:


Note: After repairs, use the scan tool Fuel Trim Reset function to reset the long-term fuel trim to 128 (0%).

  1. Fuel pressure – An excessively low pressure will make the system lean. Monitoring fuel pressure while driving the vehicle at various road speeds and/or loads may become necessary here.
  2. Map sensor - An output that enables the ECM to sense a manifold pressure which is  less than normal (high vacuum) can drive the system  lean.In such a case disconnecting the MAP sensor will permit  the ECM to substitute a fixed (default) value for the MAP sensor. If disconnecting the sensor kills the lean condition then substitute a known good sensor and recheck.
  3. Fuel contamination - In trace amounts, water , near the in-tank fuel pump inlet can be added to the injector. water results in  a lean exhaust condition and can set DTC P0171.

Check for poor O2S or MAP sensor connection at the ECM. Inspect the harness connectors for the following conditions:

  • Improper mating
  • Damaged terminals
  • Broken locks
  • Improperly formed or
  • Backed-out terminals
  • Poor terminal-to-wire connection


·         Also examine the wiring harness to check whether there is any damage.

·         If the harness seems flawless, then note the O2S display on the scan tool as you move the connectors and the wiring harness related to the engine harness.

·         Any difference in the display will denote the location of the fault.




·         You must check the 5 volt reference circuits to inspect whether a short to voltage is in vogue in case you see any DTC P1404 is set.

·         You also need to see if there is any restricted exhaust system.

·         Probe the 5 volt reference sensors for abnormal readings in case a shorted 5 volt reference circuit creates a DTC P0172 to set.