P0121 OBD Error Code: Problem & Solutions

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Located on the Throttle Body of the Intake Manifold is the Throttle Position Sensor/Switch and attached as part to the accelerator Pedal Assembly is the Pedal Position Sensor/Switch. Both of these sensors give exact entry from the driver’s foot according to how much power the engine requires and how urgently the power is needed.

While the Throttle Position Sensor is rotating by its base that rests its position to full speed, it usually sends an enhanced voltage signal to the Powertrain Control Module (PCM). The PCM uses the voltage signal that increases or decreases to control the Air Fuel Ratio and Spark Timing of the engine, including other Emission Systems Components.

What causes this problem with the Throttle Position Sensor/Switch A Circuit?

• TPS has irregular open or short circuit that run internally
• Harness is rubbing and prompting an open or short circuit in the wiring
• TPS has a bad connection
• PCM is bad (less likely)
• Water or corrosion is found in connector or sensor

1. Boost pressure sensor - If the boost pressure sensor is defective, the amount of boost power will start depleting and the engine’s efficiency will decline. A damaged boost pressure sensor should be replaced to stop a turbocharged car in losing efficiency in the engine.
2. Turbocharger - Low oil levels may lead a turbocharger to fail, at an extent that excessive heat and friction will finally disintegrate the bearings that are inside. In this case, a turbocharger needs to be replaced or the parts must be examined for efficient movement by using internal lubricants.
3. Supercharger - A supercharger can begin to fail for all sorts of different reasons, for instance a supercharger oil of low level, a broken pulley, and seals/gaskets that are too hot or defected. If the supercharger in your car is not working correctly, you need to replace it as soon as possible.
4. Knock sensor - When detecting a knock sensor, make sure the terminal is not corroded or the wires and connections are not damaged. A defective knock sensor effects how your car performs, consumes fuel and damages your engine at a certain expense. In this case, it’s important to get a specific knock sensor that is just right for your car make and performance.
5. Spark Plug - A defective spark plug may happen whenever the acceleration is slowed, power is at loss, fuel economy is low, engine misfires, and the car is difficult to start. In conclusion, it is well recommended to replace your spark plug for every 30 months or 30,000 miles that your car is traveling.
6. Intake manifold - A leaking intake manifold can pose harmful effects on your car's engine. This leads the manifold into drawing in extra air through and from the outside of the leak. As a result, excess air enters the intake ports and impedes with the air-fuel ratio in the cylinders. The faulty manifold should be replaced right away to prevent any further damage to your car's engine.
7. Boost Pressure Gauge - If the boost pressure sensor is defected, the boost power will be reduced and engine’s efficiency will decrease. In this case, you should replace the boost pressure sensor to keep your turbocharged car running with stronger and perpetual efficiency.
8. Powertrain Control Module - A defective PCM may cause your car to fail in starting or stalling when running. Moreover, poor emissions will take place as a result of failing to control the emissions system, so if your PCM is unable to function properly, it should be immediately replaced.
9. Engine Control Module - Your car may have trouble starting, especially after ensuring that the control module, it should be immediately replaced to keep your car from facing more severe difficulties.
10. Wastegate - If more exhaust gas is transferred through the turbine, the subdivision can over-accelerate in speed, posing mechanical stress on it and can cause all sorts of problems, for instance breaking shafts, defective compressor wheels, or an electrical rush to the engine's internals and power delivery. It’s appropriate to modify the turbo on its own and replace an upscale version with one customized for the task, if possible.

The P0121 code leads the engine ECM land on a failsafe mode, thus causing a wide range of drive complaints prior to when repairs are accomplished. These symptoms rise as a slow acceleration, high idle, bucking/jerking, speed limits or stalling, which all depends on the maker’s failsafe mode challenge.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination (Inspect Engine Light or Service Engine Soon Light)
• Irregular stagger on acceleration or deceleration
• Gusts black smoke on acceleration
• No start conditions

The powertrain control module (PCM) provides a 5 Volt reference signal to the throttle position sensor (TPS) and normally it’s at a ground as well. A general correction is as follows:

• If having a scan tool to access, see what functionality the idle and WOT (wide open throttle) readings bring for the TPS. Detect if they're close to the identifications brought up from above.
• Detect for an irregular open or short voltage within the TPS signal. To achieve this, you should
  not use a scan tool. What you'll need is an oscilloscope. The reason this occurs is because
  scan tools take up samples of many different readings just over one or two data
  lines and can miss an irregular drop out. It is best to hook up the oscilloscope and watch for
  the signal. It should brush up and down nicely without dropouts or spikes.
• If you aren’t noticing problems, then you should carry out a wiggle test. You can achieve
  this by wiggling the connector and harness as you are watching the pattern. Find out if it successfully drops out? If it’s successful, exchange (TPS) throttle position sensor and re-detect.
• If you don’t see a (TPS) throttle position sensor showing a signal, then check at the connector for 5 Volt references. If the signal is presently in view, detect the ground circuit for open or shorts.
• Be definite that the signal circuit is not 12 Volt. It shouldn’t ever have a battery voltage.
  If this is the case, trace the circuit for short to voltage and fix as soon as possible.
• View for any sign of water in the connector and exchange (TPS) throttle position sensor as needed.

It’s wise to always check for technical service bulletins (TSB) in advance for your particular car. You may have a known issue with your engine that can easily be sought out by the manufacturer to fix that can save you time and money during this diagnosis. It is common practice to use a diagnostic scanner, (DVOM) a digital volt/ohmmeter and an exact car data source (ALL DATA DIY) to diagnose the code P0121. Reading and recording the codes retrieved and visualizing the freeze frame data to detect when the P0121 code was set.

•  Erasing the OBD-II fault codes and re-evaluating the car to see if the P0121 code returns.
• Check and see all TPS connections and wiring for any loose or damaged items.
• Comparing the data between TPS circuit A and B on the scanner and if not correct, operate the TPS pinpoint tests according to each of the manufacturer's proposals.

Common Mistakes When Diagnosing OBD Code P0121

• Neglecting to first manage a visual check up
• Neglecting to scan, record, erase or re-examine for codes to confirm failures
• Exchanging a TPS sensor before tests detect an issue
• Failing to compare TPS sensor A and B identifications before and after repair or replacements