Troubleshooting

DTC P0172 SYSTEM TOO RICH (FUEL TRIM)

CIRCUIT DESCRIPTION

DTC Detection Condition:

"Fuel" trim is released to the feedback compensation value, not to compared against the basic injection time. Fuel trim includes short-term fuel trim and long-term fuel trim. "Short-term fuel trim" is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal theoretical value. The signal from the heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air-fuel ratio is RICH, and an increase in fuel volume if it is LEAN. "Long-term fuel trim" is overall fuel compensation carried out long-term to compensate for continual deviation of the short-term fuel trim form the central value due to individual engine differences, wear over time and changes in the usage environment. If both the short-term fuel trim and long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL lights up.

HINT:
- When the DTC P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 is recorded, the actual air-fuel ratio is on the RICH side.
- If the vehicle runs out of fuel, the air-fuel ratio is LEAN and DTC P0171 is recorded. The MIL then comes on.
- If the total of the short-term fuel trim value and long-term fuel trim value is within ± 25 %, the system is functioning normally.
- The heated oxygen sensor (bank 1 sensor 1) output voltage and the short-term fuel trim value can be read using the TOYOTA hand held tester or OBD II scan tool.

Wiring Diagram:

CONFIRMATION DRIVING PATTERN
a. Connect the TOYOTA hand-held tester to the DLC 3.
b. Switch the TOYOTA hand-held tester from normal mode to check mode.
c. Start the engine and warm it up with all accessory switches OFF.
d. Drive the vehicle at 50 - 65 kph (31 - 40 mph) for 1 - 3 min. to warm up the heated oxygen sensor.
e. Let the engine idle for 1 min.
f. Perform steps (c) to (e) 3 times.

HINT: If a malfunction exists, the MIL will light up during step (1).

NOTE: If the conditions in this test are not strictly followed, detection of the malfunction will not be possible. If you do not have a TOYOTA hand-held tester, turn the ignition switch OFF after performing steps (c) to (f), then perform steps (c) to (f) again.

Step 1 - 5:

Step 5 (Contd.) - 8:

Step 8 (Contd.) - 12:

INSPECTION PROCEDURE

HINT: Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool, as freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.

CHECK FOR INTERMITTENT PROBLEMS
TOYOTA hand-held tester only:
By putting the vehicle's ECM in check mode, 1 trip detection logic is possible instead of 2 trip detection logic and sensitivity to detect open circuits is increased. This makes it easier to detect intermittent problems.
a. Clear the DTC.
b. Set the check mode.
c. Perform a simulation test.
d. Check the connector and terminal.
e. Handle the connector.