I get this fault code 121 (KOEO), no check engine light when it's running. I checked the voltage for the power source, it gets 5V to the sensor. Then I checked the voltage for the ground and throttle position wires. When it is closed, I got 0.9V, When I opened up all the way to the max, I got 3.0V. After an hour, I rechecked my code again, no more code 121. Maybe it is not this throttle position sensor.

The problem I have is when the car is moving I rev up the rpm to 2000, it will then rev up more by itself (to 4000 or so) even I let go of the gas, then it will come back down to normal idle when the car stops moving. Engine surging is the problem. I suspect MAF sensor is the cause.

Any advice? Please help.

 

Spec for TPS is 0.69 to 0.99v when closed
4.5 to 4.8 when open all the way

P0121 Throttle/Petal Position Sensor/Switch A Circuit Range/Performance Problem

So Computer is not seeing high enough voltage when it has calculated throttle should be close to Wide Open(WOT), i.e. its only 3volts, should be 4.5volts

Since its correct when closed, 0.9v, then it could be your throttle has limited travel, something is preventing it from moving to WOT
OR
The TPS is damaged

 

Nothing is preventing it to open wide.

RonD, may I ask what would happen if the computer only sees 3 volts instead of 4.5? Will the engine be surging? This looks like my problem. Just curious.

 

Computer would not "trust" TPS data(voltage) if its "out of range", so wouldn't use TPS data
Could manifest as surging I suppose
There is no cause and effect relationship with most sensors, symptoms can be varied when any sensor is showing limited or false data

But you know for sure TPS data is in correct............so I would start there by replace it
Then move on

 

New part installed. The car would not start unless I stepped on the gas a little (I guess it opened the throttle plate a little bit). Once started, if I let go of the gas, it will die.

I re-checked the voltage on the new part,
1.08V when closed
4.72V when all the way open

Do I have a bad part or I installed it wrong (lol)? Please advise.

Btw, this is Walker Products 200-1021 Throttle Position Sensor for $16

 

Reads like IAC Valve is unplugged or bad
IAC Valve gives the computer control to allow more air in for start up, and then set the air flow for idling high(cold) or lower(warmed up)
It accurate +/- 4rpm so very precise setup

With key on engine off a volt meter plugged into both wires should show 10-12volts
IAC Valve gets the full 12v with key on(red wire) but the computer controls its Ground wire to open and close it by varying the voltage on the ground side
Computer opens IAC Valve all the way with key on, for start up, then starts to close it once engine RPMs are above 400
You can try cleaning it

You can ONLY use a Motorcraft or Hitachi IAC Valve on the Rangers, other brands are not true Solenoids, they are Stepper motor/solenoid combos, so unreliable in this apllication

 

So even I get 1.08V when closed (TPS), is it ok? or it has to be below 1V?

 

No, it should be below 1volt but not really your issue
Someone??? may have adjusted the Anti-Diesel screw on the throttle linkage, it "looks like" an idle screw but is not

When an engine is stalling people often adjust this screw which opens throttle plate a bit more but ALSO raise up TPS throttle closed voltage

You can re-adjust it now if you want, re-set so voltage is 0.9v and wide open 4.5v or higher

The TPS sensor doesn't do much, what its there for is to give instant acceleration when you step on the "gas" because the MAF sensor would be too slow, as soon as computer see a voltage increase it *** more fuel quickly and change spark timing for the richer mix
And then when your foot is off the "gas" the computer can shut off injectors while you coast to save fuel

TPS is also a "learned" sensor, computer would accept 1.08v as closed throttle if that's as low as it ever went, and anything above 4.49v is considered wide open to computer
Spec is 0.69v to 0.99v for closed throttle

TPS rarely causes running issues, except for a hesitation on acceleration, TPS replaced the "Accelerator Pump" on a carb

 

Problem resolved. It looks like the IAC wasn't connected securely. I did not mess with the screw or anything. 1.08V when closed and 4.72V when all the way open.

Thank you RonD, you are the BEST.

 

Good stuff