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The 4HP22 transmission can be rendered undriveable when held on P or N setting and too high engine rotation speed is applied for too long, for instance during emissions testing. Examples of situations that could lead to transmission failure include warming up the catalytic converter on petrol engine-powered vehicle with too high RPM, or while measuring amount of smoke on diesel engine-powered vehicle and keeping the throttle full for too long time.
The correct procedure would be to stop the engine for at least 90 seconds, do the test (without selecting again D or R, this means without re-pressurizing the gearbox) for no longer than a minute, and if the procedure must be repeated, stop again for at least 90 seconds and repeat.
Another issue is the governor valve sticking, which can be caused by contamination. i.e. clutch plates or other parts disintegrating. The fine debris finds its way past the filter and tends to accumulate in the governor, causing it to stick. A temporary solution is to remove and clean the governor. The problem will often recur as debris from damaged parts continues to build up in the governor. If the problem continues after cleaning the governor then it may be necessary it replace the autobox.
Usual symptoms are failing to shift after 1st gear while the gearbox is still cold, and then failing to kickdown to 1st while at a stop. The governor is located at the end of the gearbox and usually it starts to work again after a cleanup.