The Throttle position sensor is a potentiometer that measures the amount of
throttle opening. As the throttle is opened, the reading (measured in volts)
goes up. The powertrain control module (PCM) supplies a 5 Volt reference signal
to the throttle position sensor (TPS) and usually a ground also. A general measurement
is: at idle = .5 Volts; full throttle = 4.5 Volts. If the PCM detects that the
throttle angle is greater or less than it should be for a specific RPM, it will
set this code.
Symptoms of a P0121 trouble code could include:
- Malfunction Indicator Lamp (MIL) illumination (Check Engine Light or Service
Engine Soon light)
- Intermittent stumble on acceleration or deceleration
- Blows black smoke on acceleration
- No start
A code P0121 may mean that one or more of the following has happened:
- TPS has intermittent open or short internally
- Harness is rubbing and causing an open or short in the wiring
- Bad connection at the TPS
- Bad PCM (less likely)
- Water or corrosion in connector or sensor
1. If you have access to a scan tool, see what the idle and WOT (wide open
throttle) readings are for the TPS. Check if they're close to the specifications
mentioned above. If not, then replace the TPS and re-check.
2. Check for an intermittent open or short in the TPS signal. To do that, you
can't use a scan tool. You'll need an oscilloscope. The reason is because scan
tools take samplings of many different readings over just one or two data lines
and can miss an intermittent drop out. Hook up your oscilloscope and watch the
signal. It should sweep up and down smoothly with no drop outs or spikes.
3. If no problems were noticed, perform a wiggle test. Do this by wiggling
the connector and harness while watching the pattern. Does it drop out? If so,
replace TPS and re-check.
4. If you have no TPS signal, check for 5 Volt reference at the connector.
If it's present, check the ground circuit for open or shorts.
5. Make sure the signal circuit isn't 12V. It should never have battery voltage.
If it does, trace circuit for short to voltage and repair.
6. Look for any water in the connector and replace TPS as necessary.
- Engine Light ON (or Service Engine Soon Warning Light)
- Lack/Loss of power
- Engine rough idle
- Large vacuum leak
- Leaking air duct system
- Mass Air Flow (MAF) poor electrcial connection
- Manifold Absolute Pressure (MAP) poor electrcial connection
- Faulty Mass Air Flow (MAF) sensor
- Faulty Manifold Absolute Pressure (MAP) sensor
- TP sensor not seated properly.
- Faulty Throttle Position (TP) sensor.
- Dirty throttle body
When is the code detected?
Powertrain Control Module (PCM) monitors operation rationality check by
comparing sensed throttle position to mass air flow readings
P0068 DODGE Description
The Powertrain Control Module (PCM
monitors a vehicle operation rationality check by comparing Throttle
Position (TP) to Mass Air Flow (MAF) readings. If, during a self-test,
the comparison of the TP sensor and MAF sensor readings are not
consistent with the calibrated load values, the test fails and a
diagnostic trouble code is stored in continuous memory.