Jeep Cherokee XJ throttle position sensor: symptoms and how to test it

The throttle position sensor sits on the side of the throttle body. The throttle shaft turns it. It’s a variable resistor: the computer feeds it about 5 volts and reads back a voltage that rises as the throttle opens.^{[1][3][4][6][8]}

The computer uses that signal to set fuel injector pulse width and spark timing at every throttle position, and to know when the throttle is closed (idle).^[3][4][8] On 4.0L automatics, the AW4’s transmission computer also reads it to choose upshift points and when to lock the torque converter.^[2][6] The 2000 manual adds that cranking with the pedal on the floor cuts fuel (clear-flood mode), and the computer knows this from the TPS.^[4] Unverified The trigger is 2.608 V above the learned idle voltage. (factory manual only)

Unverified Owners link a failing TPS to a surging or hunting idle, a stumble or hesitation just off idle, and erratic or late AW4 shifts. A worn sensor often has a dead spot partway through its travel. (owner reports; no factory symptom list) These overlap with vacuum leaks and idle air control motor problems, so test before you buy.

A failed TPS sets a code:

• 1993–95: flash code 24, TPS voltage high or low.^[1][6] Read it by turning the key On-Off-On-Off-On within 3 seconds and counting the check-engine light flashes.^[1][2][6]
• 1996–2001 (OBD-II): P0122 (voltage low), P0123 (voltage high) and P0121 (TPS doesn’t agree with the MAP sensor).^{[3][5][11][12][13]}

The factory test needs only a digital voltmeter.^[1][3][6]

1. Key on, engine off.
2. Back-probe the center terminal of the TPS connector (that’s the signal wire), meter negative to ground.^{[1][2][3][6][7]}
3. Read it with the throttle closed, then open the throttle slowly by hand to wide open.

The voltage must climb smoothly the whole way, with no jumps or dropouts.^{[1][2][3][6][7]}

For 1997–2001, the factory test passes the sensor if it reads more than 0.26 V but under 0.95 V at closed throttle and under 4.49 V at wide open.^[3][7][8]

We don’t hold a factory manual for 1991–92 or 1996, so we haven’t listed values for those years. 1987–90 Renix trucks use a different engine computer and sensor, and none of these values apply to them.

1. Remove the air intake duct at the throttle body, unplug the sensor and remove its two screws.^[5][9]
2. The throttle shaft slides into a socket with tangs on the new sensor. Fit it so the sensor has to rotate a few degrees, under slight tension, to line up the screw holes. If it won’t rotate, take it off and refit it with the shaft on the other side of the tangs.^[3][5][6][9]
3. Tighten the screws to 60 in-lb (7 N·m).^[3][5][14]
4. Work the throttle by hand to make sure nothing binds before you start the engine.^[3][5][9]

There’s no adjustment: the computer learns the closed-throttle voltage on its own.^[4][6][7]