A lit transmission warning light is the PCM telling you it detected a fault. Different vehicles use different icons — gear shape, thermometer, wrench, generic check engine. Here is what each one usually means and what to do first.
Transmission fluid temperature exceeded safe threshold (usually 240-275°F depending on platform). Causes: blocked cooler, low fluid, internal slipping generating heat. ACTION: pull over and let it cool. Check fluid level when cool. If repeated, inspect cooler and lines.
Generic transmission fault detected. PCM has logged a code. ACTION: get the code read — auto parts store will do it free. Codes in P07xx range tell you which solenoid or valve is at fault.
Many modern transmissions trigger the generic CEL for trans faults. ACTION: scan with OBD-II — look for P07xx codes specifically. P0700 means "transmission fault, look for another transmission code."
PCM has commanded transmission to stay in third gear to prevent damage. Common after sensor failure or solenoid stuck-on. ACTION: scan codes, drive only enough to diagnose. Continuing to drive can damage the converter and clutches.
Newer vehicles display text like "service transmission" or "transmission overheating." ACTION: pull codes, address as above.
Some failures (TCC shudder, soft shifts) do not trigger a light. ACTION: still scan for pending codes — many faults log without illuminating.
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