QuestionMy 2001 Elantra transmission drops into 3rd gear and will not get out of 3rd unless I stop and turn the car off. A look at the diagnostics codes indicate: P0128, P0422, P1529, P0720 and P0705. A review of your previous answer indicates that I need to change the input, output and range sensor. Should all three be changes or should I replace one at a time to see if one is causing the others to malfunction?
AnswerP0128 indicates your coolant temperature goes below the intended operating temperature. This is usually caused by using the incorrect thermostat or a stuck open thermostat, but in rare instances has been caused by a faulty coolant temperature sensor. Replacing the thermostat with a factory one gives you good chances of repairing this item.
P0422 indicates that your manifold catalyst efficiency is low. If you've got no exhaust leaks, you'll need a new manifold catalytic converter. If you have less than 80,000 miles, this should be covered by the federal emissions warranty. If you're the original owner and the vehicle has less than 100,000 miles, it should be covered by the powertrain warranty since it's part of the manifold.
P0705 indicates a problem with the range switch signal. Replacing the range switch typically cures the issue.
P0720 indicates a problem with the signal from the output speed sensor. Replacing the output speed sensor typically cures the issue.
P1529 indicates that your TCM asked the ECM to turn on the check engine lamp.
The range switch and the output speed sensors are the items that are abnormally affecting the operation of the transmission. The diagnostic logic in your computer can distinguish these issues from one another, so you'll definitely want to do both. You have no diagnostic code for the input speed sensor (P0715) listed, so I have made no recommendation to replace it. In my opinion, it's not a labor-intensive repair, so I wouldn't recommend replacing it unless you have reason to believe it's bad.
