Unfortunately a simple dash warning light is just too generic to pin down to one thing. Honestly, your first reference should be your owners manual. It will give a more accurate indication of what system might need attention.
Usually dashboard "idiot lights" will indicate you need to check some fluid level such as oil, water or brake fluid. My Mercedes alerts when the washer fluid is low. So, again, check your owners manual.
Do you have gauges? Does anything look amiss? Some vehicles can indicate when something is out of tolerance, such as worn brakes. Or perhaps even as silly as a gas cap being loose, in the case of vapor recovery systems.
Find a friend that has a OBDII scanner (available at Autozone, NAPA, etc). The code, or codes, thrown can give a better indication of what is wrong rather than someone just guessing.
The warning light just says warning. I have all original owner manuals and nothing is noted about the light.
All gauges show normal operation.
I will do as suggested and have the OBDII scanner check the fault.
It's kind of a catch-all, indicating some unusual condition exists and needs your intention. Usually means an engine computer diagnostic code has been stored, but can also include things like water-in-fuel-filter, low coolant, or similar conditions that do not generate an engine code.
Here are some discussions on other sites that describe some of the conditions that can cause that light to come on.
We have a 2003 Monaco Diplomat. On our last trip, the fan blade broke and went into the radiator, all the coolant poured out. We had to be towed to a repair shop. They replaced the radiator and fan. Now there is a warning light on. When we looked it up in the manual, it just says" Out of...
You read diagnostic codes at the engine diagnostic plug, which a '99 diesel should have. It's probably not OBD2, though - big diesels use different test standards, J1939 for recent models and J1708 on older ones. It's probably a 9-pin oval plug. You can get a wired or BT adapter for it and use a smart phone to display codes. Or buy a dedicated code reader device for it. Cummins code values are available online.
Your coach isn't on a Freightliner chassis, but this old topic may be of some help on code reading.
OK, I bought an OBDII scan tool, because quite frankly, I've never owned a vehicle that got as many "Check Engine" lights as this motorhome. The scan tool came with the standard 16 pin plug for the vast majority of post '96 OBDII compliant vehicles, but Freightliner (maybe it's Cummins) uses an...