[Solved] 1998 Volvo has high idle and code P0120

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This 1998 Volvo S70 has a problem with a high idle, a fault code P0120, a blinking arrow on the dash, and a hard shift in the transmission. Let’s dive in on how I tackled this problem with just a generic scan tool.

What does the fault code P0120 mean?

P0120 - Throttle/Pedal Position Sensor/Switch A Circuit

The vehicle’s computer has to know where the throttle plate is to calculate the correct fuel ratio for the vehicle, this is the main purpose of the throttle position sensor. When the vehicle sees the throttle plate is closed and the MPH is at 0, it knows it’s at idle. When the vehicle sees the throttle plate wide open, it knows to add more fuel.

The pictures below are of the location of this vehicle’s throttle position sensor.

The throttle position sensor is underneath the throttle body.

What can happen when the throttle position sensor goes bad?

When the throttle position sensor goes bad, it can cause the vehicle to go into a default mode. Depending on the vehicle, it can go into “limp mode” where the vehicle’s power is greatly reduced, or in this case, cause a high idle.

Let’s look at some pictures of this 1998 Volvo to see the faulty throttle position sensor on the scan tool.

The picture below shows the live data on the scan tool.

There are two items to notice in the picture:

1. The RPM

2. The throttle position sensor (Displayed on the scan tool as “TP”)

The RPM in the picture is 1020. This vehicle is idling and that RPM is too high. Depending on the vehicle, a good idle range can be from 600 - 900 revolutions per minute(RPM).

The throttle position sensor (TP) is displayed at 100%. At idle, this percentage should not be that high. The computer thinks that the throttle plate is wide open, but it clearly is not since the vehicle is idling.

What would cause the throttle position sensor to go bad?

Possible causes for a throttle position sensor code can be a wiring issue, a loose connection, a bad computer, or the throttle position sensor itself.

With a simple visual inspection, the wiring and connection to the original throttle position sensor can be ruled as good. On this Volvo, they appeared to be in good shape.

Though there are many ways one could test the computer to see if it is faulty, I decided to try a new throttle position sensor.

The pictures below are the old sensor next to the new one. Notice the o-ring on the new throttle position sensor and the o-ring that is missing on the old sensor. The old o-ring accidentally became stuck on the vehicle and I did not realize it. After about 5 minutes of wondering why the new throttle position sensor would not correctly mount to the vehicle, I finally found the old o-ring still in place. Remember to remove the old o-ring if it becomes stuck.

After replacing the throttle position sensor, I took a look at the live data again. Now the TP is showing 10.2% rather than 100%. This number is much better than before. The Volvo also idles correctly and when I test drove it, the flashing arrow on the dash and the hard shifting went away.

The picture below is the new live data from the scan tool.

This Volvo is fixed!

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