P0135
DTC PO135FRONT OXYGEN SENSOR HEATER CIRCUIT MALFUNCTION (FO2H)
Description
- The oxygen sensor is used to sense oxygen concentration in the exhaust gas. If the fuel ratio is leaner than the stoichiometric ratio in the mixture (i.e. excessive amount of air), the exhaust gas contains more oxygen. To the contrary, if the fuel ratio is richer than the stoichiometric ratio, the exhaust gas contains hardly any oxygen.
- Therefore, examination of the oxygen concentration in exhaust gas makes it possible to show whether the air/fuel ratio is leaner or richer than the stoichiometric ratio.
- The oxygen sensor has a zirconia tube (ceramic) which generates voltage if there is a difference in oxygen concentration between the inside and outside of the tube. Platinum is coated on the inside and outside of the zirconia tube for the purpose of catalysis and electrode provision. The hexagon screw on the outside is grounded to the exhaust pipe, and the inside is connected to the Engine Control Module (ECM) through the harness.
- A ceramic heater is employed to improve performance at low temperature.
- When rich air-fuel mixture is burnt in the cylinder, the oxygen in the exhaust gases reacts almost completely through the catalytic action of the platinum coating on the surface of the zirconia tube. This result is a very large difference in the oxygen concentration between the inside and outside, and the electromotive force generated is large.
- When a lean air-fuel mixture is burnt in the cylinder, oxygen remains in the exhaust gases even after the catalytic action, and this results in a small difference in the oxygen concentration. The electromotive force is very small.
- The difference in oxygen concentration changes greatly in the vicinity of the optimum air-fuel ratio and hence the change in the electromotive force is also large. By inputting this information into the Multiport Fuel Injected (MFI) Control Module the air/fuel ratio of the supplied mixture can be determined easily. The oxygen sensor does not generate much electromotive force when the temperature is low. The characteristics of the electromotive force stabilize at temperature of approximately 300 to 400°C (572 to 752°F).
DTC Detecting Condition
Two consecutive trips with fault.
1. Connect Subaru Select Monitor or the OBD-II General Scan Tool, and read data.
a. Turn Ignition switch to "OFF".
b. Connect Subaru Select Monitor or the OBD-II General Scan Tool to data link connector,
c. Turn Ignition switch to "ON" and Subaru Select Monitor or OBD-II General Scan Tool switch to "ON".
d. Start Engine.
e. Read data on Subaru Select Monitor or OBD-II General Scan Tool.
Subaru Select Monitor
- Designate mode using function key.
- Function mode: F29.
[CHECK]: Is the reading of F29 0.2 A or more?
[NO]: Go to step 2.
[YES]: Repair connector.
NOTE: In this case, poor contact in front oxygen sensor connector and Engine Control Module (ECM) connector can be the possible cause.
OBD-II General Scan Tool
- For detailed operation procedures, refer to the OBD-II General Scan Tool Instruction Manual.
2. Check output signal from ECM. (Using voltage meter.)
a. Start and idle the engine.
b. Measure voltage between ECM and body.
[CHECK]: Connector & terminal:
(B84) No. 44 - Body/1.0 Volt, or less.
[YES]: Go to step 3.
[NO]: Go to next [CHECK].
[CHECK]: Is the voltage less than 1.0 Volt while shaking harness and connector of ECM?
[YES]: Repair poor contact in ECM connector.
[NO]: Go to next step.
c. Disconnect connector from front oxygen sensor.
d. Measure voltage between ECM and body.
[CHECK]: Connector & terminal:
(B84) No. 44 - Body/1.0 Volt, or less.
[YES]: Replace ECM.
[NO]: Repair short circuit of harness between ECM and front oxygen sensor connector. After repair short circuit of harness, replace ECM.
3. Check power supply to front oxygen sensor.
a. Turn Ignition switch to "OFF".
b. Disconnect connector from front oxygen sensor.
c. Turn Ignition switch to "ON".
d. Measure voltage between front oxygen sensor connector and body.
[CHECK]: Connector & terminal:
(B18) No. 2 - Body/1O Volts, or more.
[YES]: Go to step 4.
[NO]: Repair power supply line.
NOTE: In this case, repair poor contact in connector or open circuit of harness between main relay and front oxygen sensor.
4. Check front oxygen sensor.
a. Turn Ignition switch to "OFF".
b. Measure resistance between front oxygen sensor connector terminals.
[CHECK]: Terminals:
No. 1 - No. 2 30 Ohms or less.
[YES]: Repair harness and connector.
NOTE: In this case, repair the following:
- Open circuit of harness between the front oxygen sensor connector and the ECM connector.
- Poor contact in front oxygen sensor connector.
- Poor contact in ECM connector.
[NO]: Replace front oxygen sensor.