Toyota Tacoma manuals

Toyota Corolla Repair Manual: Circuit description

The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim includes the short–term fuel trim and the long–term fuel trim.

The short–term fuel trim is the short–term fuel compensation used to maintain the air–fuel ratio at stoichiometric air–fuel ratio. The signal from the heated oxygen sensor indicates whether the air–fuel ratio is rich or lean compared to the stoichiometric air–fuel ratio. This variance triggers a reduction in the fuel volume if the air–fuel ratio is rich, and an increase in the fuel volume if it is lean.

The long–term fuel trim is the overall fuel compensation carried out in long–term to compensate for a continual deviation of the short–term fuel trim from the central value, due to individual engine differences, wear overtime and changes in the operating environment.

If both the short–term fuel trim and the long–term fuel trim are lean or rich beyond a certain value, it is detected as a malfunction and the mil is illuminated.

Dtc no.

Dtc detection condition

Trouble area

P0171 When air–fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on lean side (2 trip detection logic)
  • Air induction system
  • injector blockage
  • mass air flow sensor
  • engine coolant temperature sensor
  • fuel pressure
  • gas leakage in exhaust system
  • open or short in heated oxygen sensor (bank 1, sensor 1) circuit
  • heated oxygen sensor (bank 1, sensor 1)
  • heated oxygen sensor heater (bank 1, sensor 1)
  • efi relay
  • pcv valve and hose
  • pcv hose connection
  • ecm
P0172 When air–fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on rich side (2 trip detection logic)
  • Injector leak, blockage
  • mass air flow sensor
  • engine coolant temperature sensor
  • ignition system
  • fuel pressure
  • gas leakage in exhaust system
  • open or short in heated oxygen sensor (bank 1, sensor 1) circuit
  • heated oxygen sensor (bank 1, sensor 1)
  • heated oxygen sensor heater (bank 1, sensor 1)
  • efi relay
  • ecm

Hint

:

Monitor description

Under the closed–loop fuel control, fuel injection amounts that deviate from

Under the closed–loop fuel control, fuel injection amounts that deviate from the ecm’s estimated fuel amount will cause a change in the long–term fuel trim compensation value. This long–term fuel trim is adjusted when there are persistent deviations in the short–term fuel trim values. And the deviation from a simulated fuel injection amount by the ecm affects a smoothed fuel trim learning value which is the combination of smoothed short–term fuel trim (fuel feedback compensation value) and smoothed long–term fuel trim (learning value of the air–fuel ratio). When the smoothed fuel trim learning value exceeds the dtc threshold, the ecm interprets this as a fault in the fuel system and sets a dtc.

Example: the smoothed fuel trim leaning value is more than +35% or less than –35%, the ecm interprets this as a fail in the fuel system.

Monitor strategy

Typical enabling conditions

Typical enabling conditions

Typical malfunction thresholds

Typical malfunction thresholds

Wiring diagram

Wiring diagram

Refer to dtc p0130

System too lean/rich
Dtc p0171 system too lean (bank 1) Dtc p0172 system too rich (bank 1) ...

Inspection procedure
Hint: hand–held tester only: narrowing down the trouble area is possible by performing ”a/f control” active test (heated oxygen sensor or other trouble areas can be distinguished). Perform ac ...

Other materials:

Emission inspection and maintenance (I/M) programs
Some states have vehicle emission inspection programs which include OBD (On Board Diagnostics) checks. The OBD system monitors the operation of the emission control system. If the malfunction indicator lamp comes on The OBD system determines that a problem exists somewhere in the emission cont ...

Circuit description
The ignition switch provides circuit power and the ecm provides the circuit ground that illuminates the mil. Mil operation is checked visually: it should be illuminated when the ignition is first turned on. If the mil is off all of the time or on all of the time, use the procedure below to ...

Inspection procedure
1 Check airbag sensor assy center connector Disconnect negative (–) terminal cable from the battery, and wait at least for 90 seconds.  check the connection of the airbag sensor assy center connectors. 2 Check air bag sensor assy center Sst 09843–18040 Connect the negat ...

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