2000 Mitsubishi Eclipse TPS code issue

Fault codes never say to replace parts or that they're bad. They only indicate the circuit that needs further diagnosis, and that's where you are now.

There's three wires and three voltages to check, but they're only accurate and correct if the sensor remains plugged in. You'll have to back-probe through the rubber seals in the plug. The important one is the signal wire in the center. If that is correct, the other two have to be correct.

One wire is the 5.0 volt supply. The ground wire will have 0.2 volts. The signal wire will have approximately 0.5 volts at idle and about 4.5 volts at wide-open-throttle. This is the wire and these are the voltages that set the fault codes related to this sensor.

P0121 - Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem

Unfortunately this code doesn't get very specific so we're going to have to go by what you find for voltages.

Measure them with your voltmeter. 0.2 volts on the ground wire, 5.0 volts on the feed wire, and roughly 0.5 on the signal wire at idle.

Those voltages are perfect. The signal voltages are approximate, but for discussing theory of operation, I use 0.5 volts at idle and 4.5 volts at wide-open-throttle. In practice you might find something like 0.38 and 4.22 volts. The point is, it can only get all the way to 0.0 volts or 5.0 volts when there's a break in the circuit, and it's those voltages that trigger a fault code to set.

When you read the correct signal voltage at the sensor but a fault code is still being set, there's two things that can do that. Most commonly there's an intermittent problem that isn't acting up while you're measuring voltages. The most likely suspect is corroded or stretched terminals in the sensor's connector. Next would be a wire rubbed through and touching a spot on the engine or body sheet metal where the paint is rubbed off. The least likely suspect is a break inside the sensor itself. That can happen, especially on Ford sensors, but for others a break usually is permanent and easy to find.

The second thing is a break in the signal wire. You will still measure the proper range of voltages at the sensor, but the computer will see a constant 5.0 volts. That's what you'd see on a scanner's display too, and that will set the fault code. With that break, the voltage on the signal circuit inside the computer could "float" to some random value due to being interconnected with a lot of other internal circuitry. If those random voltages were to fall within the 0.5 to 4.5 volt range, the computer would accept them as correct and try to run on them. To prevent that, a "pull-up" resistor is used to put 5.0 volts on that circuit and force a defective condition to be detected and set a fault code. That pull-up resistor is so big electrically that when there is no defect, it has no affect on the circuit's normal operation.

The point is, if a fault code continues to set when the voltage you measure at the sensor is correct, you need to look at the live data display on a scanner to see what the computer is seeing. By watching the TPS voltage on the scanner, you can also wiggle wire harnesses and connectors to see if you can find an intermittent connection.