Diagnosis & Repair

WARNING: This page is about a different car, the 2003 Toyota 4Runner. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Using Toyota hand-held tester or scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.

For diagnosis and repair procedure, see Fig 1-Fig 7 .

For ECM location, see Figure. For ECM connector view, see Figure. To identify circuits referenced in testing, see ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS article in ELECTRICAL. For vacuum or EVAP hose routing, see VACUUM DIAGRAMS article. For testing of systems or components, see BASIC DIAGNOSTIC PROCEDURES - V6 & V8 article or SYSTEM & COMPONENT TESTING - V6 & V8 article. For adjustments, see ON-VEHICLE ADJUSTMENTS - V6 & V8 article. For removal and installation of components, see REMOVAL & INSTALLATION - V6 & V8 article.

Fig 1: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 1-4)

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 2: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 5-8)

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

NOTE: At this point in diagnosis, review the following list of hints while referring to Fig 3.

NOTE: In the following list, the voltage in parenthesis are as seen on OBD II scan tool. The voltages outside parenthesis are as seen on Toyota hand-held tester.

Whenever the output voltage of the A/F sensor remains at approximately 3.3 volts (0.66 volt) under any conditions as well as the above conditions, the A/F sensor may have an open circuit. (This will happen also when the A/F sensor heater has an open circuit). See drawing in Fig 3.
Whenever the output voltage of the A/F sensor remains at a certain value of approximately 3.8 volts (0.76 volt) or more, or 2.8 volts (0.56 volt) or less under any condition as well as the above conditions, the A/F sensor may have a short circuit. See drawing in Fig 3.
The ECM will stop fuel injection (fuel cut) during engine deceleration. This will cause a lean condition and should result in a momentary increase in A/F ratio sensor voltage.
The ECM must establish a closed throttle position learned value to perform fuel cut. If the battery terminal has been disconnected, the vehicle must be driven over 10 mph to allow the ECM to relearn the closed throttle position.
When the vehicle is driven, in the case that the output voltage of the A/F sensor is below 2.8 volts (0.56 volt) during fuel enrichment, (for example, when the vehicle tries to overtake another vehicle on a highway, the vehicle speed is suddenly increased with the accelerator pedal fully depressed), the A/F sensor is functioning normally.
The A/F sensor is a current output element, and therefore the current is converted into voltage inside the ECM. If measuring voltage at connectors of A/F ratio sensor or ECM, you will obtain a constant voltage.
If voltage is not as specified, proceed to step 10). See Fig 4-Fig 6 . If voltages are as specified, go to step 17). See Fig 6 & Fig 7 . When it is necessary to perform CONFIRMATION DRIVING PATTERN, see TEST DRIVE CONFIRMATION .

Fig 3: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Step 9)

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 4: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 10-11)

Fig 5: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 12-13)

Fig 6: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 14-18)

Fig 7: DTCs P0171, P0172, P0174 & P0175 Diagnostic Procedure (Steps 19-22)