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Truck running choppy, P0120 code being thrown...

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Hey guys, forgive me if this topic has already been covered.

 

My truck is doing two "strange" things. The first is every now and then, the engine dies and all of the idiot lights on the dash light up. This happens for about 1/4 second, and then engine resumes and the lights go out.

 

Then yesterday, it started idling choppy and the Check Engine Light came on. When I got home, I scanned for codes and got P0120, which the online OBD resources say is:

 

"P0120 is the generic OBD-II code which indicates that the Engine Control Module (ECM) has seen the TPS sensor circuit A output voltage going below or above the sensor's expected range, or is chattering when compared to TPS sensor circuit B."

 

I am guessing this indicates that something is wrong with my TPS?

 

The truck is a 1998 Pathfinder, automatic tranny.

 

Thank you in advance for any guidance!

 

 

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Could be the TPS, the plug, the wiring, or something else entirely. The codes just give you a starting point, still have to put in the work and diagnosis to actually find the problem. 

 

It is possible that a poor electrical connection at the battery, fusible links, ground, or a failing alternator could cause false codes to set because the ECU is simply seeing voltage on the circuit that is outside of what is allowed. With your other symptoms you have described, I would personally start by ensuring the electrical system is good. Most of your local auto parts stores can perform a battery and charging test in the parking lot and don't charge for it. Try wiggling wiring and see if anything is loose or damaged. Check for corrosion at terminals and connectors. Wiggle things with the engine running and see if it glitches. I had a gremlin in my 1980 200sx long ago that drove me nuts. Took a dark night for me to see the Sparks where the fuel injection harness ground wire screw was just a little loose. Cleaned and tightened the ground wire and the problem went away, making room for so many others. That car hated my wife, it was smarter than me there. 

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agreed use a multimeter or take it to a auto parts store sounds like a alternator starting to go

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Thanks, Mr_Reverse. I appreciate your reply.

 

I *should* have prefaced the situation with what I know/have done.

 

Load tested battery, it specs out *better* than its rating.

 

I tested voltage at battery with the truck running, and the alternator is delivering rated output (I would like to point out that there are clunks and squeals and some grinding noises coming out of the alternator occasionally that disturb me, but I am never up on the battery terminals when this happens to see if the charging current changes).

 

I have jiggled every wire, terminal, relay and fuse that I could touch. I have roughly jiggled both of the relay boxes under the hood.

 

I have replaced the ground cable with a factory new unit. I have the factory positive cable, and I will install it today.

 

 

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I have a brand new Denso/Bosch starter, and I will be installing it today as well, since I have to get down there for the far end of the positive cable, anyways. The positive cable does not look like a lot of fun to install.

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Strange noises out of the alternator is a red flag. Sounds like brushes or bearings are about done. I can tell you that my old 93 is sensitive to dirty and low power from a failing or bad alternator. Newer stuff is even more so. Voltage testing is ok for basic checking, but current testing is also required for alternators. It can put out good voltage, but low current. Need to use the AC voltmeter setting to check how clean or dirty the power is.

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Thank you, Mr_Reverse!

 

I will work on current testing alternator, but, since its making those noises, I am likely to replace it and the idler pulley on general principle.

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