P0143

CIRCUIT DESCRIPTION
The VCM supplies a voltage of about 0.45 volt (450 mV) between the HO2S High and HO2S Low circuits. (If measured with a megaohm digital voltmeter, this may read as low as 0.32 volt.) The Heated Oxygen Sensor (HO2S Bank 1 Sensor 1) varies the voltage within the range of about 1.0 volt (1000 mV) if the exhaust is rich, down to about 0.10 volt (100 mV) if the exhaust is lean. This DTC is a type B DTC.

The Sensor is like an open circuit and produces no voltage when it is below 360°C (600°F). The DTC P0143, P0144 or an open sensor circuit or the cold sensor causes the Open Loop operation. The DTC P0143 determines if the HO2S sensor or the circuit is shorted to low by checking for a lean condition during a steady throttle and power enrichment (PE).

CONDITIONS FOR SETTING THE DTC
The following conditions will set the DTC:

^ No TP sensor DTCs.
^ No Evaporative Emissions (EVAP) DTCs.
^ No IAT sensor DTCs.
^ No MAP sensor DTCs.
^ No ECT sensor DTCs.
^ No MAP sensor DTCs.
^ No intrusive test in progress.
^ No device controls active.
^ The system voltage measures at least 9.0 volts.

Test Enable Lean Test

^ Closed Loop.
^ No fuel injectors turned off.
^ Air to fuel ratio is at least 14.5 but less than 14.8.
^ The TP is greater than 5% but less than 99%.
^ The above met for 5 seconds.
^ No HO2S (Bank 1 Sensor 3) DTCs.
^ HO2S Bank 1 Sensor 3 voltage is less than 0.026 volts (26 mV).

Test Enable PE Lean Test

^ A Closed Loop.
^ The Power Enrichment mode active.
^ The high speed fuel cutoff not active.
^ HO2S (Bank 1, Sensor 3) DTCs not active since code clear.
^ Time elapse since test enable is at least 1 second.
^ The Bank 1 Sensor 3 HO2S voltage measures less than 0.399 volts (399 mV).

ACTION TAKEN WHEN THE DTC SETS
A current DTC P0143 set causes the system to operate in an Open Loop. With a DTC P0143 set, the MIL (Malfunction Indicator Lamp) turns on after 2 consecutive failures.

CONDITIONS FOR CLEARING THE MIL/DTC
The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

DIAGNOSTIC AIDS
Using the scan tool, observe the L.T. fuel trim values at the different RPM and the air flow conditions. The scan tool also displays the fuel trim cells, so the scan tool can check the L.T. fuel trim values in each of the cells in order to determine when the DTC may have set. If the conditions for this DTC exist, the L.T. fuel trim values measure around 158 or greater.

Check for the following conditions:

^ A heated oxygen sensor wire: A sensor pigtail may be mis-positioned and contacting the exhaust manifold.
^ An intermittent ground in wire between the connector and the sensor.
^ A poor VCM to the engine block ground.
^ Lean injectors: Perform the Poppet Nozzle Test. Refer to Fuel Injector Balance Test.
^ Fuel Contamination: Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can also set this DTC.
^ Fuel pressure: If the pressure is too low, the system will be lean. In order to confirm, monitor a fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Pressure Test.
^ Exhaust leaks: If there is an exhaust leak, the engine may pull the outside air into the exhaust. and past the sensor.
^ Vacuum or the Crankcase leaks can cause a lean condition or a possibly a high idle.
^ If the above check out okay, the problem is a faulty heated sensor.

Never solder the HO2S wires. For proper wire and connector repair refer to Wiring Repairs.

TEST DESCRIPTION
The numbers below refer to the step numbers in the diagnostic table.

2. Running the engine at 1200 RPM keeps the oxygen sensor hot in order to ensure the oxygen sensor remains active and can indicate the exhaust oxygen content accurately.
3. Opening the heated oxygen sensor circuit should result in a displayed voltage between 350 and 550 mV. If the display is still fixed below 350 mV, the fault is a short to ground in the HO2S signal circuit or a faulty VCM.