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Diagnostic codes

Car has 70,000 mi. Recently had a fault code p0735 come up indicating an incorrect gear 5 ratio. Dealer says trans. input/output speed sensor is defective. What is the relationship between this sensor and a gear ratio? Speed sensor fix is not under warranty, but another shop says the code should indicate something else internal. What is the speed sensor and why would that trigger the gear 5 code? Could it be something else internal and how do I find that out without dropping too many dollars running from shop to shop?

Disclaimer: These comments are general, and not specific to your car as I know little to nothing about Kia.

The transmission has speed sensors at various places. If the computer knows how fast the input shaft is turning, how fast the output shaft is turning, and what gear the transmission is in, it can tell if it’s working correctly. The error code means that the speed change from the input to the output is not correct for the gear selected. It could be a sensor issue, but this is unlikely. If one of the speed sensors was defective, then you should be getting errors for all the different gears. Since you are only getting an error for 5th gear, it seems likely that you have a fault in the transmission related to 5th gear.
No one can really say what the problem is just from the code. There are troubleshooting procedures to follow to determine what exactly is wrong. Someone will have to do that. There is a possible exception. If it turns out that over 90% of the Kias that have this code have a defective sensor, then it would be reasonable to skip the troubleshooting and just replace the sensor.
Maybe someone with specific Kia experience will chime in.

Thank you SO much. This issue has been driving me crazy and keeping me up nights. Trying to understand the innards of a tranny is mind-boggling. When you say they will have to troubleshoot, is that beyond doing the diag. check and getting the codes? The dealer is telling me flat out the sensor is defective, but that’s external to the powertrain warranty. If the real problem is inside, then I want to find out obviously so it would be covered. Thanks again.

I’m not sure what you mean by “diag. check”, but certainly they need to do more than a computer scan to retrieve the code(s). Usually (but not always), that would involve opening the transmission.

sorry, yes, what I meant was troubleshooting beyond plugging in the meter to get the codes. I spent about 2 hours at the dealership, but not sure if that’s enough time for them to open up the transmission. (but I’m not the mechanic either).

Two hours should be enough time to determine if it is really just the sensor or something more serious. Of course, that assumes that they worked on the car for at least an hour of those two hours you were there.
You might try to negotiate with them. If they are 100% percent certain that have have correctly identified the fault, then they should be willing to guarantee that they will fix the problem for the amount quoted, with no additional cost to you. This worked to my benefit once at a Lincoln dealer that was sure that a simple pad change would fix the brake problem I had. I told them that it wouldn’t because I had already tried that. I paid for a pad change, and they ended up (at their expense) replacing the calipers, rotors, and master cylinder to fix the issue.

ouch on your brake issue. I have decided to get a 3rd opinion at an Aamco. They too seem to agree it’s something more internal. wish me luck.

I’d stay away from Aamco. From what I’ve seen, if you don’t need a transmission when you go in, you probably will when you come out.

Does your car DRIVE OK?? Can you detect any change in the way the car drives or shifts?

At least demand this from ANY repair shop. “Will you guarantee that this repair, at the price you quoted, will cure my problem?” Don’t authorize any work until you get a yes answer…

If they “know” what is wrong, make them stand behind their work…If they don’t “know” what is wrong, don’t let them work on your car!!

Speed sensors can be checked with a good high end scanner. Usually when a speed sensor is bad you will get multiple gear ratio codes along with either an input or output speed sensor code. Since you are only getting a 5th gear code I’m leaning towards an internal problem.

Input and output speed sensors work together to tell the computer when to shift the transmission. The computer takes the turbine (Input) speed of the transmission and the output speed and computes them into a gear ratio. When the computer reads the ratio coming from the sensors compared to pre programmed ratios in the computer the transmission will shift. If one of these sensors are faulty the computer will be receiving false information which will confuse the computer. The computer then sets the trouble code/s. I really think you need to get this checked by a trans tech because It would surprise me if one of the speed sensors are causing the problem only in 5th gear.

Here are pictures of your input and output speed sensors.