The check engine light came on in my 2003 Jeep Liberty so I took it to Autozone to have it checked by their computer. It showed that the throttle position sensor might need to be replaced. Is this urgent? What kind of problems might it cause? How much should I expect to pay to have it replaced?
Fault codes never say to replace parts. They only indicate which circuit needs further diagnosis. Wiring problems account for about half of the problems that set sensor codes.
Throttle position sensors are relatively inexpensive and are an easy do-it-yourself project, but they don't fail very often. You can get a better idea of what's wrong with the circuit from the exact code description. Did it say "voltage too high", "voltage too low", etc.
March, 24, 2012 AT 5:44 PM
It said "Probable cause: Sensor open or shorted internally - replace component & Check connector and wiring to sensor"
March, 24, 2012 AT 5:59 PM
The code was P0123 if that helps.
March, 24, 2012 AT 6:29 PM
Dandy. The sensor is fed with 5.0 volts, and the movable contact has mechanical stops to limit its travel from about 0.5 volt at idle to about 4.5 volts at wide-open-throttle. It can't go beyond those voltages unless there's a break in a wire, a break in an internal connection, or an intermittent contact with the movable contact.
The code description I have is "signal voltage too high". In the case of an intermittent contact, there is a "pull-up" resistor in the computer to force the signal voltage to go to a known defective state so it will set the appropriate code. You could indeed have an intermittent sensor, but the best thing to do first is to inspect the connector pins in the plug, be sure there's no sign of white or green corrosion, reconnect it, and erase the code to see if it comes back. Since this is a relatively low-level code, I think the Check Engine light will be off after you restart the engine until / unless the fault occurs again. Most codes of that type self-erase after 50 engine starts if the problem doesn' occur again.
The throttle position sensor has probably the least effect on fuel metering of all the sensors so replacement is not real critical. What's more important is the computer constantly monitors lots of other things and they all have criteria that must be met before those tests are performed. If the computer has set a code for the throttle position sensor, it likely won't perform self tests that occur at certain throttle positions, or for things that are compared against the throttle position sensor's readings. That lack of self-testing has two concerns. The first is the vehicle won't pass an emissions test if you live in an area where that is required. Second, a totally different problem could arise that has a bigger affect on how the engine runs, but the computer won't detect that problem and set a code to tell you where to look. Other than that, the engine could run just fine.
Here's a photo from rockauto. Com to show what the sensor looks like. Two bolts go through the brass rivets to hold it to the throttle body assembly.