P0121 is a relatively common trouble code with the Toyota Mirai. It’s an OBDII code and has to do with the Throttle Pedal Position Sensor. It is relatively serious problem since it can leave your vehicle in failsafe mode.
P0121: Throttle Pedal Position Sensor → Switch (TPS) A Circuit Range → Performance Problem
OBDII code P0121 is a serious problem that requires serious attention. Diagnosing it can be tricky without a good scanner and the knowledge of what you are doing. There’s a YouTube video below that shows how to go about diagnosis with a scanner.
P0121 will trigger in your Toyota Mirai if it’s TPS “A” circuit has an output voltage that goes above or below the sensor’s normal operating range. It’s a general powertrain code, which means that it has the same meaning regardless of who manufactured the vehicles (1996+).
Toyota Mirai P0121 Symptoms
The symptoms of P0121 are going to vary by year and model, but they certainly are going to cause noticeable changes in the way that your Mirai drives. It’ll go into failsafe mode.
Here are the most common symptoms of P0121:
- The vehicle will hardly respond to the throttle at all. This limited power is a failsafe to keep the vehicle able to move, but not super fast. Since the ECM won’t really know how much throttle response is actually happening when you push the gas.
- The vehicle’s ECM will control the amount of fuel entering the engine in order to control engine speed.
- The Mirai may idle differently.
Failsafe mode leaves a vehicle in a state that makes it almost impossible to drive and enjoy.
P0121 Causes- Mirai
In the Toyota Mirai, the P0121 is going to be caused by:
- A discrepancy between the voltage reading from TPS sensor circuit “A” and TPS circuit “B” where TPS circuit “A” is higher than the specified voltage range by more than 1.6 volts.
- A discrepancy between the voltage reading from TPS sensor circuit “A” and TPS circuit “B” where TPS circuit “A” is lower than the specified voltage range by more than 0.8 volts.
In normal English, the two TPS circuits can’t agree on how far the gas pedal has been pushed down. The computer thinks that the culprit is the “A” circuit. This is what you need to address.
Toyota Mirai P0121 Diagnosis
A good mechanic will utilize a scanner to diagnose when the codes were set and reset them. They’ll then see if the problem comes back.
The mechanic will also compare the data from TPS circuit “A” and “B” to see where the discrepancy is. After that they’ll typically inspect the TPS wiring to make sure that it is not loose or damaged.
If it all checks out and the code comes back, they may go ahead and replace the TPS sensor in question. Most of the time, the problem when P0121 is thrown will be the TPS sensor itself. It no longer is accurately tracking how hard the fuel pedal is being pushed.
Diagnosing the issue with a scanner or having a mechanic take a look can save the expense of buying a TPS sensor that your Mirai does not actually need.
Good luck diagnosing P0121 in your Toyota Mirai. If there is anything that you would like to add, please leave a comment below.