P0121
***THIS ARTICLE HAS BEEN UPDATED BY TSB # 61-65-44 DATED 9/96***
Circuit Description
The Throttle Position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The signal voltage will vary from less than I volt at closed throttle to about 4 volts at Wide Open Throttle (WOT). The TP signal is one of the most important inputs used by the PCM for fuel control and most of the PCM-controlled outputs. The PCM monitors throttle position and compares actual throttle position from the TP sensor to a predicted TP value calculated from engine speed. If the PCM detects an out of range condition, DTC P0121 will be set.
Some 1997 vehicles built prior to September 15, 1996 may illuminate the Malfunction Indicator lamp and store Diagnostic Trouble Code P0121 in error. Refer to TSB #67-65-32.
Conditions for Setting the DTC
^ No TP or MAP sensor DTCs set.
^ The engine is running.
^ The idle air control valve position is between 0 and 130 counts.
^ The MAP reading is less than 50 kPa (TP sensor high test) or greater than 70 kPa (TP sensor low test).
^ The throttle angle is steady.
^ The predicted throttle angle (based on engine speed) is not close to actual throttle angle.
^ The above conditions are present for at least 10 seconds.
Conditions for Clearing the MIL/DTC
^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.
Diagnostic Aids
The Scan tool will not display a passed status for the DTC P0121 diagnostic test until the PCM completes both the stuck high (high TP angle during a low MAP situation) and the stuck low (low TP angle during a high MAP situation) portions of the test. When verifying a repair or otherwise attempting to monitor the DTC P0121 diagnostic, refer to Conditions for Setting the DTC for the MAP signal levels required to complete both parts of the diagnostic.
Check for the following conditions:
^ Vacuum hoses disconnected or vacuum leaks. Visually and physically check for the following conditions:
- Disconnected vacuum hoses.
- Damaged vacuum hoses.
- Incorrect vacuum hoses routing.
- Damaged MAP sensor seal.
- Damaged fuel injector 0-rings.
- Damaged gasket at the throttle body, intake manifold, PCV valve cover, EGR pipes, or EGR valve mounting surface.
- Incorrectly installed or malfunctioning PCV valve.
^ Damaged or sticking throttle plate or IAC valve. A throttle plate or IAC valve that allows too much airflow with the throttle closed may cause a high idle and DTC P0121 to set. Refer to Idle Air Control System Check for diagnosis.System Check - Idle Air Control (IAC )
^ Incorrect 3X reference signal. A condition that causes an incorrect 3X reference signal may cause the MAP sensor value to stop updating. This condition can cause DTC P0121 to be set during a closed throttle condition. An incorrect 3X reference signal should cause DTC P1374 to be set. If DTC P1374 and DTC P0121 are set, diagnose DTC P1374 first.
^ Stuck open EGR valve. An EGR valve stuck partially open may cause the TP sensor low test to fail and DTC P0121 to set. A sticking EGR valve should cause DTC P1406 to set. If DTC P1406 and DTC P0121 are set, diagnose DTC P1406 first.
^ Restricted exhaust system.
^ Skewed MAP signal or malfunctioning MAP sensor. An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value during high engine load or closed throttle situations. Check for the following conditions:
- an unusually low MAP reading with the key ON, engine not running.
- an unusually high MAP reading at idle. Either condition can cause DTC P0121 to set.
^ Poor connection at the PCM. Inspect harness connectors for the following conditions:
- Backed out terminals.
- Improper mating
- Broken locks.
- Improperly formed or damaged terminals.
- Poor terminal to wire connection.
^ Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the malfunction.
If DTC P0121 cannot be duplicated, the information included in the Fail Records data can be useful in determining vehicle mileage since the DTC was last set. If the DTC occurs intermittently, performing the DTC P1122 or DTC P1121 Diagnostic Charts may isolate the cause of the malfunction.
Test Description
Number(s) below refer to the Step number(s) on the Diagnostic Table:
2. An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value during high engine load situations. Check for an unusually low MAP reading. This condition can cause DTC P0121 to be set.
12. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.