* Low or No Transmission Fluid: This is the first and easiest thing to check. Low fluid can cause various shifting problems, including the inability to engage forward gears. Check the fluid level (with the engine running and warmed up) and the condition of the fluid (it should be red and clear, not brown or burnt).
* Forward Clutch Pack Failure: The most common cause of this symptom is a problem with the forward clutch pack. This pack of clutches engages to allow the transmission to move forward. If it's worn, burnt, or damaged, it won't engage properly. This requires a transmission rebuild or replacement.
* Valve Body Problems: The valve body controls fluid flow within the transmission. A malfunctioning valve body, including sticking or broken valves, can prevent the forward clutches from engaging while allowing reverse to work. This often needs professional diagnosis and repair/replacement.
* Low Input Shaft Speed Sensor (ISS) issues: Although less likely to cause *only* forward gear failure, a faulty sensor could misinform the transmission control module (TCM) about the engine speed, preventing proper gear selection. This needs diagnostic checking.
* Transmission Control Module (TCM) Failure: A faulty TCM could prevent it from sending the correct signals to engage the forward clutches. Diagnosis is required to determine this, usually via a scan tool.
In short: While low fluid is the easiest and cheapest thing to check first, the problem almost certainly lies within the transmission itself. It's highly unlikely to be a simple fix and will most likely require a professional transmission mechanic to diagnose and repair or replace the transmission. Simply adding fluid won't solve a clutch pack problem; it might mask a bigger issue temporarily.
