Error Code P0121 is defined as Throttle Pedal Position Sensor/Switch (TPS) A Circuit Low Input.

This error code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

Definition

Error Code P0121 means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined that the TPS (Throttle Pedal Position Sensor/Switch) circuit A output voltage is lower than its expected range (0.2V or less). For some vehicles, the limit is usually at around 0.17V – 0.20V.

Common Symptoms

As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. This code also puts the vehicle into “fail-safe mode”, which cuts the current to the throttle actuator and put the throttle valve at less than 6˚ of opening, which means there will be very limited throttle response and very slow speed for the vehicle. Other symptoms include:

  • Rough ridding
  • Low idle
  • Surging
  • Stalling
  • No or little acceleration

Possible Causes

Error Code P0122 means one or more of the following has happened:

  • Short to ground TPS circuit or another wire
  • TPS is improperly mounted
  • Defective TPS
  • Faulty PCM (rare)

How to Check

As with other error codes, mechanics diagnose this code by using a scan tool to scan and document the codes and view the freeze frame data when the code was set.

Then, the technician will clear the code and retest the vehicle to see if the code comes back.

Next, the technician will conduct a visual inspection of the TPS connection and all of its wirings to check for damaged or loose items.

Then, the technician will compare the data between the TPS circuits A and B on the scanner, and if they’re incorrect, they will then perform the TPS pinpoint test as specified by the manufacturer.

Depending on the diagnosis, TPS may be replaced. Codes must be cleared before taking the vehicle for a road test again.

How to Fix

Repairs for this cod can be simple and straightforward, which of course depends on the findings of the diagnosis. Common repairs include:

  • Repair or replacement of the TPS connector
  • Repair of the wiring as necessary
  • Replacement of the TPS with a new sensor

Since this code puts the vehicle into fail-safe mode, which means the vehicle cannot be driven since it lacks acceleration, has limited speed, buckles and jerks, has high idle, or will stall anytime. This is to keep the vehicle from being driven and minimize further damage. Thus, this problem must be addressed as soon as possible.

When replacing the TPS, it’s important that it’s what is causing the problem. Also, the TPS sensor A and B must be compared to the specifications before and after the repair or replacements.