If a vehicle won't downshift into passing gear, what is a likely cause?

When a vehicle won't downshift into passing gear, a common cause is an issue with the throttle cable or linkage being out of adjustment. The throttle cable connects the accelerator pedal to the transmission, signaling when to shift gears based on the driver's input. If this cable is misadjusted or has developed slack, the transmission may not receive the correct signal to execute a downshift when the driver accelerates for passing, leading to a lack of response.

While the malfunctioning governor valve can influence shifting behaviors, it primarily affects the timing of shifts rather than preventing a downshift entirely. A worn 2-3 shift rail would also typically not prevent downshifting, but rather could cause issues with shifting between those specific gears. Thus, while both these components can lead to shifting problems, they are less likely to directly cause the inability to downshift into passing gear compared to an adjustment issue with the throttle cable or linkage.