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P2121 Throttle Position Sensor D Circuit Range/Performance

Article by
Dale Toalston
ASE Certified Technician

OBD-II Trouble Code Technical Description

Throttle/Pedal Position Sensor/Switch "D" Circuit Range/Performance

What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.

The Throttle position sensor is a potentiometer that measures the amount of throttle opening. As the throttle is opened, the reading (measured in volts) goes up.

The powertrain control module (PCM) is the main computer controlling the vehicle and it supplies a 5 Volt reference signal to the throttle position sensor (TPS) and usually a ground also. A general measurement is: at idle = .5 Volts; full throttle = 4.5 Volts. If the PCM detects that the throttle angle is greater or less than it should be for a specific RPM, it will set this code. The "D" refers to a particular circuit, sensor, or area of a particular circuit.

Potential Symptoms

Symptoms of a P2121 trouble code could include:


A code P2121 may mean that one or more of the following has happened:

Possible Solutions

1. If you have access to a scan tool, see what the idle and WOT (wide open throttle) readings are for the TPS. Check if they're close to the specifications mentioned above. If not, then replace the TPS and re-check.

2. Check for an intermittent open or short in the TPS signal. To do that, you can't use a scan tool. You'll need an oscilliscope. The reason is because scan tools take samplings of many different readings over just one or two data lines and can miss an intermittent drop out. Hook up your oscilliscope and watch the signal. It should sweep up and down smoothly with no drop outs or spikes.

3. If no problems were noticed, perform a wiggle test. Do this by wiggling the connector and harness while watching the pattern. Does it drop out? If so, replace TPS and re-check.

4. If you have no TPS signal, check for 5 Volt reference at the connector. If it's present, check the ground circuit for open or shorts.

5. Make sure the signal circuit isn't 12V. It should never have battery voltage. If it does, trace circuit for short to voltage and repair.

6. Look for any water in the connector and replace TPS as necessary.

Related P2121 DTC Discussions

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