How to Diagnose a Throttle Position Sensor Code Like a Pro

Why Your Car Has a Throttle Position Sensor Code and How to Fix It

Quick Summary
Mechanical and electronic throttle systems use two internal throttle position sensors that work inversely to validate each other. A proper diagnosis requires an understanding of how both sensors share a 5-volt reference and ground, and how the ECM tracks the inverse signals to detect a fault. Once you understand reference voltage and ground, it’s easy to diagnose these common throttle position sensor codes:
P0120 – Throttle/Pedal Position Sensor “A” Circuit
P0121 – Throttle/Pedal Position Sensor “A” Range/Performance
P0122 – Throttle/Pedal Position Sensor “A” Circuit Low Input
P0123 – Throttle/Pedal Position Sensor “A” Circuit High Input
P0220 – Throttle/Pedal Position Sensor “B” Circuit
P0221 – Throttle/Pedal Position Sensor “B” Range/Performance
P0222 / P0223 – TPS “B” Low or High Input
P2135 – Throttle/Pedal Position Sensor Voltage Correlation

Article

How I Diagnose a Throttle Position Sensor Code Without Guessing

Over the years, I’ve seen many misdiagnosed throttle bodies. Someone pulls a throttle position sensor code, sees reduced engine power or limited RPM, and immediately replaces the throttle body. Sometimes that works. Sometimes it doesn’t. And when it doesn’t, now you’re chasing wiring or ECM problems after spending hundreds of dollars.

Electronic throttle control systems are drive-by-wire systems and, as such, have built-in redundancies for safety. In other words, they don’t use a single throttle position sensor; they use two (typically find TPS1 and TPS2), and the ECM constantly checks both to see if the data from each sensor correlates.

If data doesn’t correlate, the ECM sets a throttle position sensor code. If both fail, the vehicle often goes into limp mode, limiting engine RPM.

Understanding How Dual TPS Signals Work

The ECM provides a 5-volt reference to both sensors, and both sensors share the same ground. However, their signal outputs are different (opposite) — and that’s where the magic happens.

For example, in most applications, when the throttle plate is closed:

• TPS1 outputs a low voltage (typically under 1 volt)
• TPS2 outputs a high voltage (typically over 4 volts)

As the throttle opens:

• TPS1 voltage increases
• TPS2 voltage decreases

They are inversely proportional. And if you add the TPS1 and TPS2 voltages together, the total should be very close to 5 volts at all times. Here’s an example.

At closed throttle, TPS1 reads about 0.8 volts, and TPS2 reads about 4.2 volts, for a total of about 5 volts. The actual total varies slightly due to the voltage drop in the wire and connectors. Then, at wide-open throttle, TPS1 may read about 4.2 volts, and TPS2 may read about 0.8 volts, for a total of about 5 volts.

That inverse relationship is critical for correctly diagnosing a throttle position sensor code.

Step 1: Check Live Scan Data First

With the key on and the engine off, read the TPS data on your scan tool. At closed throttle, I expect TPS1 voltage to be low and TPS2 voltage to be high. Then, as I slowly press the accelerator, I want to see:

• TPS1 voltage steadily increasing
• TPS2 voltage steadily falling

As long as you’re pressing the accelerator in a smooth motion, you shouldn’t see any dropouts, spikes, or flat spots in the readings. If the voltage changes are smooth and remain inversely proportional, the system is likely functioning properly.

If not, conduct further testing with a multimeter. NEVER condemn the throttle position sensors based on scan data alone, because the problem could be in the wiring or connectors.

Step 2: Verify 5-Volt Reference and Ground with a Multimeter

Most throttle position sensors have three wires: 1 for the reference, 1 for the ground, and 1 for the signal to the ECM. If you have the wiring diagram for your vehicle, great; otherwise, you can decipher the wiring setup with just your multimeter.

Typical throttle position sensor wiring diagram

Set your meter to 20 volts DC. With the key ON and the engine off, connect one meter lead to a good ground, then backprobe the TPS connectors until you see 5 volts. That’s the reference voltage.

Next, check for ground. Leave the ground meter lead connected to ground. Set your meter to read ohms. Then backprobe each of the other two wires to find the one that shows a resistance value, not OL or ∞ (infinity).  Obviously, the remaining wire is the signal wire to the ECM.

Start by verifying the 5-volt reference. Without that reference voltage, neither sensor can generate a signal, and you will absolutely trigger a throttle position sensor code.

With the key on and engine off, backprobe the 5-volt reference wire. It should read close to 5 volts. Next, verify good ground on the ground wire.

If the reference voltage is off by more than you’d expect from normal voltage drip, suspect a bad connection or damaged wire between the throttle position sensor and the ECM.

Step 3: Check TPS Signal Voltage at the Connector With a Multimeter

With a known-good reference voltage and a good ground, I move on to measure the output signals directly at the throttle body.

At closed throttle, I expect to see TPS1 around 0.5–1.0 volts and TPS2 around 4.0–4.5 volts. Then, I slowly open the throttle and monitor both voltages. I’m looking for three things:

• Smooth voltage change
• Inverse movement
• Combined total near 5 volts

If I see clean signals at the throttle body but incorrect readings on the scan tool, the problem is likely in the wiring between the throttle body and ECM. If the signals are bad right at the throttle body, then the throttle body assembly is the culprit.

What Causes a Throttle Position Sensor Code?

In my experience, a throttle position sensor code can be triggered by:

• Internal TPS failure
• Corrosion in connector terminals
• Damaged signal wiring
• Loss of 5-volt reference
• Poor ground circuit
• ECM internal fault (rare, but possible)

Wiring issues are common, especially near the throttle body, where heat and vibration take their toll. Another common cause is intermittent signal dropouts. Even a momentary glitch can trigger a throttle position sensor code, sending the vehicle into reduced power mode.

When diagnosing a throttle position sensor code, I always remind people:

The ECM controls the throttle motor. The TPS sensors report position back to the ECM. The ECM constantly cross-checks both signals.