Engine/Transmission (1998.5 - 2002) 0121 Code - Whats it mean?

[mongoose]

Got an 0121 Code on my '99 Truck.



Erratic Idle

Long Starts,

Check engine Light Occasionally



This is what I found on the Code Description:



"P0121 (M) TPS Voltage Does Not Agree With

MAP

TPS signal does not correlate to MAP sensor signal. "



Where should I start?

 

[Gary - K7GLD]

I'd start with a recalibration of the APPS (Accelerator Pedal Position Sensor), or TPS (throttle Position Sensor) in your case - hopefully adjustment is the same for both - my comments below are what's used on the APPS.



As I recall the simpler adjustment method (someone will correct me if I get it wrong - just like my wife does! )



Disconnect BOTH batteries for a half hour or so, then reconnect them.



Next, turn on ignition key, but don't start engine yet - instead, make certain there is nothing blocking FULL depression of the accelerator pedal, like floormats, etc. , then carefully and smoothly slowly FULLY depress the accelerator all the way to the floor, then do the same as your return it to the top of it's travel.



That's it! This exercise establishes the resistance range of the APPS to the ECM, and generally coordinates signal and data syncronization between the two.



If the APPS is WAY out of adjustment, here's the more involved alignment procedure as supplied by another TDR member:

RESETTING THE APPS SENSOR.

(ACCELERATOR PEDAL POSITION SENSOR)

2ND GENERATION 24 VALVE TRUCKS ONLY.



LIST OF TOOLS NEEDED



DIGITAL VOLT METER

# 20 TORQUE BIT SOCKET

# 1 PHILLIPS

# 10 MM SOCKET



Proper service and repair procedures are vital to ensure personal safety of those performing the repairs. Standard safety procedures and precautions should be followed at all times to eliminate the personal injury or improper service, which could damage the vehicle or compromise its safety.

Although this material has been prepared with the intent to provide reliable information, no liability is assumed in reliance of this material.



It has come to my attention that a lot of people think by disconnecting & re-connecting the batteries and stepping on the throttle it is going to reset the apps, no, that is not correct. It is impossible to adjust your apps if you do not know where it is supposed to be set at. That is why you need the voltmeter.



This is the procedure I use to re-set the apps.



With the key on, engine off you need to probe the apps wire to see where your voltage is currently set at.

The best location to probe the wire is on the PCM (power control module) which is located off the passenger side of the firewall.



· You want the C1 connector, this is the connector closest to the engine,

· You want the orange wire with the dark blue tracer which is pin # 23

· Voltage should read somewhere around 0. 5 volts,

· At this point the voltage reading does not matter, it just has to be accurately written down for later reference.



*** TURN THE IGNITION OFF ON THE TRUCK ***



· The apps sensor is located slightly above and a little to the left of the injection pump.



· Remove the black plastic cover that is located by your injection pump.



The two screws that are holding it are plastic, do not put any downward pressure on them or you will never get them off.

· Un do the 6 - 10 mm headed bolts that hold the bracketry in place.

(DO NOT REMOVE THE CABLES)



You will notice the apps is on the back of the bracketry that you just removed, it is held in place by 2 - 20 bit screws. These screws have a little bit of locktite on them so make sure you have a good socket and gently give a little tap with a hammer before attempting to loosen. Be very careful these screws strip very easily and you only get one shot at them.



· On the apps you will find a white tag that gives you the information on what the apps adjustment should be.



· After loosening the screws you can rotate the apps both clockwise and counter clockwise to get the adjustment you need.



· The reading you took at the beginning of the process on pin # 23 should match the white tag on your apps, if not adjust accordingly.

 

[mongoose]

Thanks, I'll Give it a Try!