HO2S - Handling and Diagnostic Procedures: Overview
17/01/96/US12/31/96
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 H02S operates on a principle similar to temperature sensors. However, unlike like the temperature sensors, the H02S switches quickly as the 02 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 on the following pages 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 (H02S) as outlined below. Then if necessary, perform the snap throttle test on Upstream sensors.
PARTS INFORMATION
ERR1834 - Heated Oxygen Sensor
WARRANTY
Normal warranty policy and procedures apply.
COMP. CODE
SRO
19.22.16: H02S renew front - 0.35 hr
19.22.16.01: H02S renew rear - 0.35 hr
19.22.96: TestBook hook-up, access codes, complete DTCR, 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.
PROCEDURE
IMPORTANT: 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.
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,
CONNECTOR INSPECTION
When a sensor fault is stored but the sensor appears to operating normally, it would be useful to check the HO2S wiring. Refer to the Operation Pride Manual for the Discovery and Range Rover circuit diagrams.
IMPORTANT: Use extreme care when handling the sensor. Do not drop or physically damage the sensor. If sensor has been dropped, do not install on vehicle.
REMOVAL AND INSTALLATION
1. Remove any dirt or debris in the vicinity of the sensor to avoid contamination in the catalytic converter or the threads in the exhaust pipe.
2. Verify vehicle is not at operating temperature.
3. Remove old sensor.
4. Apply anti-seize to the threads of the new sensor, taking care that no anti-seize contaminates the sensor tip.
5. Install sensor and finger-tighten in the exhaust pipe.
IMPORTANT: Proper torquing of the sensor is critical. Experience has shown that if the sensor is over-torqued, there is a possibility that the body of the sensor element can distort, causing a cracked sensor element. Use the proper tool and the proper torque specification.
6. Using special tool # LRT - 12-O47, tighten sensor to 20 Nm, (15 lb.ft).
Retest After Repair
1. Clear any codes that may have been stored with Testbook.
2. Perform drive cycle C. (Refer to service bulletin W-96-007).
2. Recheck with TestBook for any newly stored codes.
3. If codes were stored, proceed with the Snap Throttle test.
After retrieving oxygen sensor P-codes from GEMS, go to real time monitoring oxygen sensors/fueling and check voltage activity of sensors as well as sensor heaters. If you observe that the voltage is fixed at 4.8 or 1.0 volts. then rev the engine several times quickly and see if the voltages begin switching. A sensor that does not respond will be defective or have a connection problem. Thus it is unnecessary to proceed with the oxygen sensor "snap throttle" test below.
If the sensor is not responding, check the heater operation. The oxygen sensor heaters maintain the tip of the sensors at their optimum operating temperature. The two front heaters are driven in parallel as are the two rear heaters. The heater voltages shown in Real Time Monitoring are used by the ECM to check the temperature of the heaters. when the engine is warm, these voltages will settle to approximately 1 volt.
NOTE: A voltage of approximately 0.5 volts could be caused by one open circuit heater. Follow the circuit checks to diagnose open circuit
IMPORTANT: A good sensor must meet all these specifications, provided the procedures were followed exactly as Outlined. If the engine idles too long, the sensor may cool down too much. This can lower its maximum voltage and increase its response time, causing a possible test failure. Many sensors will perform adequately when very hot (after 2-3 minutes at 2500 rpm), but will fail when cool. Others may fail at high temperatures. Check in both conditions.
SNAP THROTTLE TEST OF UPSTREAM SENSORS USING BEAR ANALYZER "UNISCOPE"
NOTE: If pinging is encountered while performing the Snap Throttle Test, poor fuel quality is usually the cause. This must be remedied before proceeding with test.
1. Calibrate lead set for accurate voltage readings.
2. Switch off A/C compressor.
3. Set Uniscope red channel on ± 7 VDC and 32 ms.
4. Back probe pin 33 of red 36 pin connector for bank B, or pin 34 for bank A sensors.
5. Run engine to operating temperature (approx. 195°F) then heat engine and HO2S's fully by running engine 2-3 minutes at 2500 rpm. Let idle for 5 seconds but no more than 20 seconds.
6. Push record on Uniscope.
7. Rev the engine rapidly four to five times in 2 second intervals from idle to wide open throttle (do not exceed 300 rpm).
8. Examine recording frame by frame. set cursor # 1 at maximum HO2S voltage and take a reading; record maximum voltage results. Set cursor # 2 on minimum HO2S voltage and record results.
9. Measure rich-lean response times.
NOTE: Do not judge response times at the beginning of the record cycle (i.e. frames 1-3).
10. Print screens using print icon on upper right corner of screen.