Oxygen Sensor - Handling and Diagnosis: Overview
12/31/9617/01/96/US
SUBJECT:
PRIDE - HO2S: Handling and Diagnosis
MODEL:
Range Rover 4.0/4.6
Discovery
Discovery
AFFECTED VEHICLES:
LP SA302084 onwards
LH
LJ TA166228 onwards
LJ TA502787 onwards
DETAIL
The Heated Oxygen Sensor (HO2S) is used to supply the ECM with information used to modify the operation of the fuel and ignition system for the best economy and emission performance. The HO2S measures the amount of oxygen in the exhaust stream as an indication of combustion efficiency. When the engine is running rich there is very little oxygen in the exhaust, and vice versa.
The HO2S operates on a principle similar to temperature sensors. However, unlike like the temperature sensors, the HO2S switches quickly as the O2 content of the exhaust changes. The speed of switching is a measure of the serviceability of the sensor. The GEMS sensors have a separate heater circuit. The sensor will not operate correctly if the heater is not working. Power for the heaters is supplied whenever the key is in the ON position. The ground side of the heater circuit is controlled by the ECM and is switched on and off while the engine is running to maintain a consistent sensor tip temperature.
When oxygen sensor P-codes are retrieved from TestBook, the Bear Analyzer's Uniscope function can be used in determining if these oxygen sensor P-codes were caused by the sensor or something upstream of the sensor (i.e. engine, mechanical or fuel system problems, etc.). when no faults are found after diagnosis of the system, the Snap Throttle test will show if the sensor is capable of operation in normal driving conditions. If the sensor proves good with this test you should look for the problem elsewhere (i.e., an exhaust leak).
NOTE:
The snap throttle test is applicable to upstream sensors only. However, the downstream sensors can be diagnosed if they are moved into the upstream position.
The diagnostic steps and repair procedures detailed in this bulletin were developed as an element of the Land Rover Way OPERATION PRIDE. This TIB provides a library location for PRIDE information within the Land Rover North America technical literature system.
ACTION REQUIRED
If oxygen sensor P-codes are retrieved, thoroughly check the four heated oxygen sensors (HO2S) as outlined below. Then if necessary, perform the snap throttle test on upstream sensors.
PROCEDURE
The HO2S sensors used on Land Rover GEMS equipped vehicles are very fragile. It is important that the proper handling guidelines are observed.
The most common sensor used is zirconium based and generates a voltage of up to 1 volt based on the difference between the oxygen in the exhaust stream and that of the atmosphere.
Land Rover vehicles use a titania based sensor that operates differently. Instead of producing a voltage, the sensor changes resistance in response to oxygen in the exhaust. A rich mixture creates a low resistance in the sensor and vice versa.
A five volt reference signal is supplied to the sensor. In the case of a rich mixture, the low resistance in the sensor "pulls" the voltage low. Conversely, a lean (high oxygen content) mixture will cause the sensor to have high resistance and a high voltage. The typical range on a GEMS equipped vehicle is between less than 1 volt to more than 4 volts.
NOTE:
Older vehicles have a three wire sensor that operates on a similar principle with different wiring and different voltage ranges.
WARRANTY:
Normal warranty policy and procedures apply.
COMP.CODE:
SRO:
19.22.16: HO2S renew front - 0.35 hr
19.22.16.01: HO2S renew rear - 0.35 hr
19.22.96: TestBook hook-up, access codes, complete DTC, and diagnostic drive cycle C (service bulletin W96-007) - 1.25 hrs
19.26.26: TestBook access codes, Uniscope analysis, DTCR, & drive cycle C
19.22.89.29: Snap Throttle test - 0.25 hr
* A 0.05 hr time allowance can be claimed for each P-code found using the appropriate SRO code. (Ex. If P-code 1137 is stored, use SRO 19.22.1137.
DIAGNOSIS
1. Attach TestBook and pull Diagnostic Trouble Codes (DTC).
2. Access Freeze-Frame data. Fill out DTCR and FAX to LRNA at (301) 731-9814.
3. Check the operation of the sensors using Multiple Real Time Monitoring on TestBook.
4. Start engine and set engine to about 2500 rpm for 2 or 3 minutes.
NOTE 1:
When the catalytic converters start to operate as the engine warms up, the downstream sensors will not react the same as the upstream sensors if the converters are operating properly.
NOTE 2:
A fixed voltage of 4.99 volts indicates an open circuit in the sensor or sensor circuit.
5. Observe HO2S switching activity. The sensors should be switching rapidly between 0 volts and 5 volts. Go to Oxygen Sensors/Fuel screen for a faster reading from individual sensors if necessary.
6. If the sensor is not responding, check the heater operation. The heaters are wired in upstream and downstream pairs. A bad heater will result in a lower voltage displayed in real time. Follow circuit checks below to diagnose.
7. Replace any sensor which is not switching properly.
INSPECTION
1. When sensor voltages do not switch or appear intermittent, the ECM harness connector terminal may have become loose.
2. Disconnect harness and carefully examine the connector terminals.
3. If they are found to be loose, release the terminal from the connector. Press down on tab with a suitable blunt object (ball point pen) enough to restore the terminal tension. While terminal is out, examine closely.
4. If any corrosion is evident, clean terminal with a suitable electrical contact cleaner.
5. If water intrusion is apparent, check connector gasketing, and repair as necessary.
6. Make sure that under body connectors are firmly held in the wire guiding clips and are not allowed to dangle loosely.
7. As in the ECM harness check, examine the connectors for corrosion or looseness.
8. While connectors are apart.