Hello!

When the "check engine" light comes on, information related to the cause is stored in the vehicle's computer memory at that time to assist the technician in diagnosing the problem later. On a vehicle this old (Pre-OBD2) this information is often limited to failure codes, but that is enough to at least determine what particular engine system is experiencing the problem. If the shop you are taking the truck to cannot begin to diagnose the system involved without the light being on, you may want to consider taking the vehicle in to a different shop that is more familiar with Nissan engine control diagnostics (perhaps the Nissan dealer would be the best choice, as they would be familiar with common failure points on your particualr vehicle). Some of these early systems were not the most user friendly, and a Nissan technician can probably diagnose intermittent faults quicker (and therefore cheaper) than someone who does not work on them everyday.

Dirty engine oil will not make the check engine light come on. A low engine oil level could cause the red oil light to illuminate, but not the amber check engine light. The check engine light is turned on by the powertrain controller when it either detects an input (sensor value) out of normal range, or an output (controlled device) not responding to commands.

If you are not noticing any change in engine operation when the light comes on, it is likely an emissions system related problem of some sort. It should still be diagnosed and corrected, as some minor faults can result in damage to other components (such as a catalytic converter) if not repaired.

I hope this is helpful to you; if so, ana ccept would be most appreciated! Thanks!

Here is a brief description of the function of the check engine light in your vehicle, from the service manual:

Images available only to Customers.

The check engine lamp, located on the left side of the instrument cluster, is used primarily to inform the driver of a malfunction in the ECU's calculation function.
It will also come on to warn the driver when a malfunction occurs that will affect the emission control system performance. Malfunctions related to emission system performance can be detected by performing MODE 3 (STORED CODES) TEST .
The lamp also flashes simultaneously with the red diode on the ECU when extracting codes or when inspecting feedback operation