One possible cause of the check engine light is a faulty oil pressure switch/sensor. The sensor is easily replaced. See this service bulletin.
The check engine light on my 1992 RRC was on when I purchased the vehicle. One of the first fixes I did was to replace the oil sensor switch. I replaced it the first time I changed the oil.
What does this light mean?
This information is from RangeRovers.net
Introduction The functions and meanings of these lights can be confusing, as they have changed over the years. These lights can easily be confused, because the "Check Engine" light serves a different purpose (emissions maintenance reminder) on early models than it does on later ones (indicating EFI faults). The emissions maintenance reminder function was taken over by the "Service Engine" light on later models, while EFI fault indication was taken over by the "Check Engine" light!!
Most Common Cause of the Light Coming On: The most common cause is one of the oxygen sensors needing replacement. (Don't bother to replace both -- the other one might soldier on for another 50K miles!). In fact, Kevin Kelly has heard from mechanics that half the EFI light and Check Engine light problems can be fixed by pulling the O2 sensor plugs apart and putting them back together with a little dab of dielectric grease. Kevin cleans all automotive connectors with spray contact cleaner (and a small rat tail file if there is any corrosion) then uses a dab of Permatex dielectric grease before putting the connectors back together.
Model Year Details and Resetting Procedures: The "Check Engine" light, located on the auxiliary switch panel, was first introduced in the 1988 model year, and for 1988-92(?) models merely reminds you that your vehicle has reached a certain mileage and should have its emissions systems checked. It is otherwise known as an "emissions maintenance reminder, and is designed to come on at 52,000 mile intervals (52,500, 105,000, etc.). It does not indicate any fault in the engine. In 1993(?) it was renamed the "Service Engine" light and performed the same function.
The light is controlled by a small brown box that looks like a relay, about one inch by two inches, with a sticker labeled "Emission Maintenance Reminder". On 1988 models it is under the lower dash panel, attached to the steering column support bracket along with some relays. On 1989 it was moved to a position under the passenger seat along with the relays behind the EFI ECU. There is a hole in the box, under the sticker, into which a small screwdriver can be inserted to connect two contacts and reset the light.
On 87-92(?) models, there is an "EFI" light in the main instrument binnacle warning light cluster. It illuminates when the ECU can no longer maintain the correct air/fuel ratio due to a malfunction in either the throttle potentiometer, mass airflow sensor, or water temperature thermistor. (Other faults can be detected by the ECU; however these do not cause illumination of the EFI light, and require the dealer's plug-in diagnostic tester to display them). Even with a fault in the system, the ECU will still get you home using its limp home" feature -- which I found indistinguishable from normal operation. The light is reset by disconnecting the battery or unplugging the ECU with the ignition off. If the light comes on again afterwards there is a definite fault. (See also Sensor Failure section of Common Problems and Fixes page).
In 1990 the 14CUX ECU was introduced, and a digital display unit was added under the passenger seat near the ECU, to display the codes for the various faults causing the EFI light to come on. To ensure we would be really confused, in 1993(?) the EFI light was renamed the Check Engine light!!
For all 1990 and up models Mike Lenaghan reports the reset procedure is different:
- Turn on the ignition.
- There is a 5 pin (or so) connector in the lead from the display. It can be found by following the lead out of the display unit - it will be somewhere else under the seat. Disconnect the lead from the display unit at the connector, and after 10 secs reconnect the lead.
- Turn off the ignition and wait until you hear the main relay click out. The next time you switch on the ignition the Check Engine light should be out. (Step 3 didn't work for Mike -- perhaps because he did not wait long enough -- so he ended up disconnecting the battery for about 1 min while the lead was disconnected and that did reset it).
Of course if you haven't fixed the problem that caused the light to turn on in the first place - it will probably come back on again!
Check Engine Light On the 4.0/4.6, the Check Engine light acts as the "MIL" (Malfunction Indicator Light) mandated under the OBD-II standard, to signal that a fault has been detected in the engine management/electronic fuel injection system. The most likely cause is the need to replace an oxygen sensor, but other fairly likely ones include the throttle positionn sensor and various other sensors. To discover what the fault is, and reset the light, you need to access the engine ECU using a scan tool such as the dealer's Testbook or an OBD-II compatible analyzer. Inexpensive versions of the latter can now be obtained for under $200. See the scan tool section for details of all these tools.
Unfortunately theGEMS models (1995-98) have non-volatile memories so the condition cannot be reset by disconnecting the battery.
Service Engine Light This light merelyy signals that the vehicle has done 82,500 miles (or a multiple thereof) and an emissions-related service is due. I had previously assumed this could be reset with a standard OBD-II scanner. However Andy Anderson recently informed me that he tried to do so on his 1996 4.0 SE using an Auto X-Ray EX Scan 1000, and found it would not do it. This was confirmed by a call to Auto X-Ray.