Circuit Description
A closed loop air/fuel metering system is used to provide the best combination
of driveability, fuel economy, and emission control. In this system, short term fuel
control is accomplished by the engine control module (ECM) rapidly responding to the
oxygen sensor readings. These changes finely tune the engine fueling. The long term
fuel trim control is determined by the trends of the short term fuel trim. The ECM
uses the long term fuel trim to make adaptation adjustments to the engine fueling.
Under normal driving conditions, except power enrichment mode and extended decelerations,
the ECM attempts to maintain a 14.7:1 air/fuel ratio. Fuel trim values of 0 percent
are ideal. If the value is above 0 percent, this indicates the engine is running
lean. Therefore, the ECM commands more fuel to the system in attempt to maintain a
14.7:1 air/fuel ratio. If the value is below 0 percent, this indicates the
engine is running rich. Therefore, the ECM commands less fuel to the system in attempt
to maintain a 14.7:1 air/fuel ratio. The ECM stores one long term fuel trim value
for each of these ranges: idle/decel and cruise/accel. The idle/decel range is known
as the additive fuel adaptation range for which only a small amount of fuel is to
be added or removed due to very low engine loads. The cruise/accel range is known
as the multiplicative adaptation fuel range for which a multiplying factor of fuel
is added or removed due to higher engine loads. DTC P0174 will set if the long
term fuel trim value for bank 2, cylinders 2, 4, 6, which
are closest to the radiator in the idle/decel or cruise/accel ranges, exceed a calibrated
rich threshold. This indicates the fuel control system has to add too much fuel (bank 2
is running lean) in order to maintain a 14.7:1 air/fuel ratio.
Conditions for Setting the DTC
DTC P0174 will set if the vehicle is in closed loop and either of the
following fuel trim values for bank 2 have been exceeded:
| • | The idle/decel is 7.7 percent. |
| • | The cruise/accel is 24 percent. |
DTC P0174 diagnostic runs continuously once the above conditions have
been met.
DTC P0174 is a type B DTC.
Diagnostic Aids
Important: Diagnose any ECT, EGR, fuel injector, IAT, heated oxygen sensor, MAF, MAP,
or TP sensor DTCs first.
Important: The LT FT BN 2 CRUISE/ACCEL (long term fuel trim bank 2) parameter on
the scan tool is only updated under moderate to heavy engine loads. As a result, between
off idle and around 50 km/h (31 mph) under light throttle, this parameter
on the scan tool will not be updated. Use the LT FT BN 2 IDLE/DECEL
parameter on the scan tool to verify the system is running lean at idle, greater than
7.7 percent or the LT FT BN 2 CRUISE/ACCEL above
50 km/h (31 mph) to verify the system is running lean during cruise,
greater than 15 percent.
Important: If DTC P0171 and P0174 are set, a problem may exist in the MAF sensor
circuit. If the MAF sensor skews towards a lower voltage, the system will be running
lean. Compare the sensor voltage to a known good vehicle under the same operating
conditions.
Other possible causes if DTC P0171 and P0174 are set:
| - | Around intake manifold, attached vacuum lines |
| - | Between upper and lower intake manifolds |
| - | At the base of the lower intake manifold |
| - | Intake manifold vacuum reservoir |
| • | The fuel pressure is low. |
| • | The fuel quality is poor or there is water in the fuel. |
| • | The EGR valve is stuck open. |
| • | The ECT is skewed towards a higher temperature. |
| • | The MAF sensor signal voltage is skewed low. |
Possible causes if only DTC P0174 is set:
| • | There is a vacuum leak on bank 2. |
| • | The fuel injectors are clogged. |
| • | There is an exhaust manifold gasket leak around the HO2S bank 2
sensor 1. |
