Code P0123 & Slipping Transmission

2002 Kia Optima
2.7 L V6 Engine, Automatic Transmission
189,000 miles

Rough Idle and a Code P0123

Recently, I noticed the check engine light (SES light) come on.  I pulled out my OBD II code reader and hooked it up under the dash.  The code which returned was P0123.  I thought that it might be a fluke as some codes will set once and never set again.  So, I cleared the code and went about my business.  But, I did notice rough idle in the car and the light came back the next time I drove the car. I used the code reader to check the code again.  P0123 was back. Now, I was certain that there was something to the code P0123 so I looked it up to see what it indicates.

Code P0123 is triggered when the voltage output from the Throttle/Pedal Position Sensor/Switch to the PCM is too high.

The Throttle Position Sensor is located on the Throttle Body of the Intake Manifold and/or the Pedal Position Sensor/Switch is located on the Accelerator Pedal. These sensors provide precise input from the driver's foot in terms of how much power is needed—and how urgently—from the engine.

As the Throttle Position Sensor is rotated from its base resting position to full acceleration, it sends a increasing voltage signal to the Power Control Module (Computer) This decreasing or increasing voltage signal is used by the PCM to manage the Air Fuel Ratio and Spark Timing of the engine as well as other emissions system components.

I immediately did some research on the internet to double check why a P0123 code is generated.  Many people suggested that the TPS does sometimes die and cause bad idle.  Others suggested a P0123 can be thrown when something else is the cause, such as a bad MAP sensor.  The part costs approx. $50 so I didn't want to just buy a new one if it wasn't bad.  So, I decided to think about the problem for a day or so before deciding how to proceed.

Can a slipping transmission really be caused by a bad Throttle Position Sensor?

The next day, something interesting happened.  As I drove away from my home in the morning, the automatic transmission slipped out of gear.  It also did not seem to want to change to the next gear.  It was driving terrible.  "Oh great!" I thought to myself.  In addition to this P0123 code, I now have a transmission which is going bad......  and then it hit me.  Maybe the P0123 code and the slipping transmission were related somehow.  Could the source of the P0123 be causing the transmission to slip?

Backyard Mechanics + Google = Best Friends

I did a search on the internet for "P0123 slipping transmission"  I read a great deal and found that others did also have a code P0123 and a slipping transmission. Queue the light bulb above my head.  I was now convinced the two were related.  The question remained, was the TPS actually bad or was there something else going on which was causing the code P0123?  Could a bad TPS actually cause the transmission to slip?  The answer to the second question is, "yes", a bad TPS can cause the transmission to slip.  The transmission depends on the signal from the TPS in order to know when to shift and how soon, etc.  If the TPS is sending errant or no data to the computer then the computer cannot tell the transmission how to shift properly. Remember, modern cars are run by computer.  If the computer receives incorrect input from an inexpensive little sensor, then the car can run very rough indeed.

A Bit of Good Fortune

When I bought this 2002 Kia Optima used in 2008, I had removed the engine and installed a new one.  I kept the old engine for parts and still have it to this day.  Being a man who hates to waste money, I began to wonder if it was it possible that the old engine in my garage had a TPS still connected to it.  I hurried to my garage and uncovered the engine.  To my glee, there was a TPS still on the old engine.  I then took that TPS off and replaced the seemingly defective one on my car with this new found one.  I then cranked the car and drove it about 3 miles.

Photos of a TPS off of a 2002 Kia Optima V6

A Happy Ending

Victory is such a sweet joy, especially when faced with a potential $1000 repair of a transmission.  To my delight, the test drive revealed that the transmission slipping had ceased and the code P0123 was gone.  The rough idle had also improved.  Darn if that tiny little Throttle Position Sensor wasn't the source of the slipping.  I realize I need a longer, more in depth test drive to be sure,  but I am firmly convinced that the P0123 code was due to a faulty TPS sensor.

In Conclusion

So, if you own a 2002 Kia Optima, or any car for that matter which has the following symptoms;

1. Check engine light showing code P0123
2. A transmission which is slipping and which began this behavior about the time the P0123 code first showed up

.... then you likely have a bad Throttle Position Sensor (TPS).  Don't take your car to the dealer or a transmission shop.  Instead, get a TPS from a junkyard or buy one new.  It is likely that replacing your TPS will fix your slipping transmission and save you a load of money and frustration.  It sure fixed mine.