The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedence DC voltmeter to the oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45), near 0.45, or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of its hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good.

If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor.

Use a high impedence DC voltmeter as above. Clamp the sensor in a vise, or use a plier or vise-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not, then the sensor is likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same as a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems.

ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always no benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.

Resetting the O2 Warning Light

First off, this reset has nothing to do with the O2 sensor itself. It is merely a maintenance "reminder" light, required by our dear government, telling you it's time to do the x0,000 mile service. It's driven by the speedo, and turns on a light on the dash after a cetain mileage has passed. In the case of Jag, the light says Ox sensor. Other brands had the light labeled "EGR" or "Service" or ...

The early cars with mechanical speedo had a Smiths unit in the middle of the speedo cable. It's a white box mounted in the engine bay at the bulkhead. This unit has an odometer looking counter that shows the percentage of mileage that has passed. It is reset by turning the recessed knob on the side until the counter shows 0000. The knob has two holes in it to turn it. Smiths makes (made?) a little key that fit. I have sucessfully reset them with a set of small circlip pliers, though this is a pain.

The later cars with electronic speedos use a different system. This is one of my favorite examples of British parts bin engineering. The box, as you mentioned is mounted in the boot on the forward bulkhead. This box is black, and has a push button to reset it. That's it. Push the white button (it is recessed behind a guard), and kerchunk, it's reset for another 30K miles. What makes this such a favorite of mine is the box. It's a VDO piece that, like the Smiths, is designed to go inline with the speedo cable. Jaguar, unlike most everyone else, did not go the all electronic route when they converted to the electronic speedo. They made a interface that takes the signal from the pulse generator, and drives a stepper motor attached to this mechanical counter.

Hey, it works... :>