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An oxygen sensor failure causes the engine to run rich, which results in high emissions. The EPA requires that
emissions systems have to work for 50,000 miles, but the 1-wire sensors needed to be replaced every 30,000 miles. To
meet the EPA requirements, up to VIN 134285 Jaguar provided a warning light on the dash that lights when the EFI
system detects a bad sensor -- right? Wrong. The light is there, but it’s only a counter that tells you that you’ve driven
30,000 miles since the last time the counter was reset -- which, hopefully, was the last time the oxygen sensors were
replaced. The “service interval counter” that turns on the light is connected to the speedometer sender, not the EFI
system. Your sensors may still be just fine when the light comes on -- or they may have given up the ghost 10,000
miles back! There are reports of the oxygen sensors failing before the counter turns the indicator on. It is
recommended that the sensors be replaced when the light tells you to, and if the car doesn’t appear to be running
correctly don’t overlook the possibility that they have gone bad even though the light hasn’t come on yet. The test of
the closed-loop operation described above will tell you if they aren’t working. Another clue: the fuel economy will
take a precipitous drop if they aren’t working, like 2 mpg or more.
The service interval counter is in the trunk, hidden under the carpeted panel on the left side near the filler cap; you need
to remove a carpeted panel in the forward left corner of the trunk to get to it. The rearward-facing end of the black
plastic box has an electrical multiconnector, a metal device that looks like a solenoid, and a single-wire connector. If
you lean your head in a little farther, you’ll see that towards the right (and hidden around the corner of the panel the
counter is mounted on) there is also a small protrusion pointing rearward with a tiny white plastic tit on top. To reset
the timer to zero when you replace the sensors, use a pointy object to push the center of this white plastic button in (a
long ways, like a half inch) until you hear the thing reset.
If all that wasn’t clear enough, you can look at pictures of the service interval counter at:
http://www.jag-lovers.org/xj-s/book/ServiceIntervalCounter.html
Another option, obviously, is to simply unplug the service interval counter. Or remove the light bulb.
Note that the service interval counter described only appears on cars with oxygen sensors up to VIN 134285 -- the end
of the 1986 model year. 3-wire sensors were introduced with the beginning of the 1987 model year, and they last
60,000 miles between replacement. Since the EPA only requires that the emissions systems work for 50,000 miles, the
service interval counter was omitted.
There was a different type of “service interval counter” behind the dashboard in the early pre-H.E. cars that obviously
has nothing to do with oxygen sensors since those cars didn’t have any.
The mid-90’s cars come with the OBD or OBD II features required by California law, and therefore have fault codes to
indicate when sensors aren’t working.
3-WIRE OXYGEN SENSORS: Starting with VIN 134286 -- the beginning of the 1987 model year -- the XJ-S came
with “three-wire” oxygen sensors with built-in heaters to keep them at operating temperature even when the exhaust is
relatively cool. Such sensors were expensive at one time, but Tomco part number 11014 is available at Discount Auto
Parts for only about $40 each. Rusty Beard found another generic: “The Part # is 341-90-06.”
Regarding wiring up the generic 3-wire sensor, Peter Cohen says, “I bought a pair of Tomco (boxed as Lucas) 3-wire
oxygen sensors. These come with 2 white wires and one black wire. The instructions say to hook black to black and
white to either of the white wires. Only problem is, there are no white wires on the Jaguar. It has one black, one G and
one NS.” Rusty Beard explains how to connect it: “Black wire to G (to ECU). The white wires to Black and NS (This
is just a heater circuit).”
RETROFITTING 3-WIRE OXYGEN SENSORS TO EARLY CARS: It’s a very good idea to retrofit the 3-wire
sensors into the earlier cars that came with 1-wire sensors. Even discounting the lower emissions, better fuel economy,
smoother idle, and other possible benefits of upgrading, installing 3-wire sensors can actually save you money; the 1-
wire sensors have a useful life of 30,000 miles, but the 3-wire sensors have a useful life of 60,000 miles -- apparently
because the heating helps prevent the buildup of deposits.