94 F150 Changed TPS Now Throws a 121 Code
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Junior User
Joined: Jun 2021
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From: Michigan
94 F150 Changed TPS Now Throws a 121 Code
Hi again,
I can't find anything in the searches on this. We had to yank the throttle body off the truck the other day, since we were already there, I changed out the TPS. There wasn't anything wrong with the original, but this truck is a bit of a project, so why not, right?
Anyway, since changing....I have a 121 code. Does it take a little bit of driving for the PCM to read the new TPS? We haven't/can't drive the truck. Just curious if I need to tank the throttle body back off.
I can't find anything in the searches on this. We had to yank the throttle body off the truck the other day, since we were already there, I changed out the TPS. There wasn't anything wrong with the original, but this truck is a bit of a project, so why not, right?
Anyway, since changing....I have a 121 code. Does it take a little bit of driving for the PCM to read the new TPS? We haven't/can't drive the truck. Just curious if I need to tank the throttle body back off.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
Assume Code 121 is a KOEO or KOER code?
If so use a meter to verify the TPS output closed throttle, key on, engine off. It should be between 0.6 to 1.0 VDC. Anything outside that range will trigger that code. There is no "learning" process per se for a TPS. Every time you start the engine the computer is sensing the lowest closed throttle TPS output then adjusts using an offset value, aka ratch algorithm. When you turn the key to Off then Run again that offset value reverts back to default and the cycle starts again.
If so use a meter to verify the TPS output closed throttle, key on, engine off. It should be between 0.6 to 1.0 VDC. Anything outside that range will trigger that code. There is no "learning" process per se for a TPS. Every time you start the engine the computer is sensing the lowest closed throttle TPS output then adjusts using an offset value, aka ratch algorithm. When you turn the key to Off then Run again that offset value reverts back to default and the cycle starts again.
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Junior User
Joined: Jun 2021
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From: Michigan
Fixed the TPS. I’m becoming fairly adept at ya king that throttle body.😂
I’m a zero codes on KOEO. and a single code at KOER. Code 159
you’ll find this amusing, after you helped me yesterday, I ordered a new PCM at RockAuto.
texted my son is ordered it..
“Oh, I figured you’d just use the one the guy gave us when we bought the truck.”
forgot about that one.
New PCM cleared my problem with pin9 and the ground.
So, I’m at code 159. I checked all of the intake tube for leaks, the only thing I’m unsure about is the IAT sensor in the filter box… I pulled it, cleaned it with MAF cleaner,
looking for a way to determine whether it actually is working or not.
Truck still running awful. Black smoke out the exhaust, hesitant throttle when gas pedal depressed.
thinking I’ll change the O2 sensor In the exhaust tomorrow as well.
I’m a zero codes on KOEO. and a single code at KOER. Code 159
you’ll find this amusing, after you helped me yesterday, I ordered a new PCM at RockAuto.
texted my son is ordered it..
“Oh, I figured you’d just use the one the guy gave us when we bought the truck.”
forgot about that one.
New PCM cleared my problem with pin9 and the ground.
So, I’m at code 159. I checked all of the intake tube for leaks, the only thing I’m unsure about is the IAT sensor in the filter box… I pulled it, cleaned it with MAF cleaner,
looking for a way to determine whether it actually is working or not.
Truck still running awful. Black smoke out the exhaust, hesitant throttle when gas pedal depressed.
thinking I’ll change the O2 sensor In the exhaust tomorrow as well.
Lead Driver
Joined: Jan 2019
Posts: 5,086
Likes: 869
From: N.W. Alabama
how to test, IAT=ACT-->Fuel Injection Technical Library » Air Charge Temperature (ACT)
how to test, ECT-->Fuel Injection Technical Library » Engine Coolant Temperature Sensor (ECT)
how to test, ECT-->Fuel Injection Technical Library » Engine Coolant Temperature Sensor (ECT)
Lead Driver
Joined: Oct 2016
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From: Houston/Sugar Land
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Junior User
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Yeah, I have the book too....Doesn't mean I know what Out Of Range Means.
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Logistics Pro
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From: Foothills of NC
Probably needs the MAF cleaned. Just spray the sensor with MAF cleaner, starting fluid, or something similar that won't leave a film. They do get dirty from time to time, especially if running an aftermarket air filter like the ones you oil up.
Lead Driver
Joined: Oct 2016
Posts: 6,185
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From: Houston/Sugar Land
New doesn't mean it's good. Did you buy a Motorcraft MAF? A bad MAF can cause the issues you mentioned in post 5. Did you have the 159 code before replacing the MAF? After you installed the new MAF, did you disconnect the the battery for 10 or so minutes so the 159 code would be cleared and the MAF functions would be relearned by the PCM?
Symptoms of a Faulty Mass Air Flow Sensor
Symptoms of a Faulty Mass Air Flow Sensor
- The engine is very hard to start or turn over.
- The engine stalls shortly after starting.
- The engine hesitates or drags while under load or idle.
- Hesitation and jerking during acceleration.
- The engine hiccups.
- Excessively rich or lean idling.
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Junior User
Joined: Jun 2021
Posts: 84
Likes: 13
From: Michigan
New doesn't mean it's good. Did you buy a Motorcraft MAF? A bad MAF can cause the issues you mentioned in post 5. Did you have the 159 code before replacing the MAF? After you installed the new MAF, did you disconnect the the battery for 10 or so minutes so the 159 code would be cleared and the MAF functions would be relearned by the PCM?
Symptoms of a Faulty Mass Air Flow Sensor
Symptoms of a Faulty Mass Air Flow Sensor
- The engine is very hard to start or turn over.
- The engine stalls shortly after starting.
- The engine hesitates or drags while under load or idle.
- Hesitation and jerking during acceleration.
- The engine hiccups.
- Excessively rich or lean idling.
I have a different thread going about the project. It's not the MAF. We've tested the MAF thoroughly. Resistance, Volts, the whole shebang. It's working. The truck currently has larger issues. You can check it out here. https://www.ford-trucks.com/forums/1...ght-track.html
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