The check engine light is the least informative sentence your truck knows how to say — somewhere between 'minor sensor sulk' and 'catastrophe in progress,' with no punctuation. Here's how to make it talk, and the honest sorting rules for keeping rolling versus shutting down.
An amber light with no code is a rumor; a code is a diagnosis-in-progress. Your options, cheapest first: many dash clusters flash codes through a stalk/button sequence (your operator's manual knows the dance), most ELDs display engine faults, a $20–$40 truck-stop code reader covers the basics, and any shop pulls the full picture for $100–$200. Even just photographing the code and searching it converts "the light is on" into "the #2 NOx sensor is lying again" — different phone calls entirely.
The honest sorting question: is the light alone, or does it have friends? A solo amber light on a truck that runs normally is a schedule-it problem. A light plus anything — power loss, new sound, smell, smoke, gauge movement, more lights — is a today problem, and the combination decides how big a today problem. Lights lie occasionally; symptoms don't.
Two closing rules from every fleet shop's wall: don't clear codes to make the light go away — you're deleting the evidence your mechanic needs, and it comes back anyway; and don't let anyone diagnose a code as a catastrophe over the phone — the authorization questions apply double when the estimate started from a lamp. Codes read, symptoms noted, vetted shop chosen: that's the whole play.
Amber generally means service soon — drive with caution and watch gauges. Red means stop safely now; continuing is how a sensor problem becomes an engine problem.
Any shop with a heavy-truck scanner can pull it in minutes, many mobile mechanics carry one, and some ELDs and dash apps read fault codes too. Know the code before you authorize anything.
If the condition stops occurring, some codes go inactive and the light drops after enough clean cycles. But a light that keeps returning is a message, not a mood.