P0121 and weird idle but not TPS?

Im 99% sure this isn't the typical TPS issue because I've tried two new ones in the last week... One cheap one and one mopar.

Jeep is a 98 4.0/AW4

Anyways, here's the story... About a month ago I washed under the hood and I'm pretty sure I got water in the TPS. A p0121 code showed so I went and bought a new cheap TPS. It worked for a week. Then p0121 showed again... Figured I'd get the mopar TPS and be done with it... But nope. It worked fine for a couple days, but now p0121 is up again.

When it does this, my jeep feels handicapped... Revs to 2k sometimes on cold idle, then when hot it doesn't want to stay idling. Transmission won't switch out of first gear, feels like power cuts out when over 2500rpm.

I am thinking maybe it wasnt actually water in the tps but rather some sort of vacuum problem?

A random but maybe related problem is that my air selector switch (to choose vents/feet/defrost) was only working intermittently earlier this month... I could choose a different setting but it would keep blowing out of the same vents. Seems to be working fine now though. But maybe this was a hint at a vacuum leak somewhere?

If not the sensor, then maybe the wires or wire plug. Check for chafed or melted insulation and corrosion.

Possible causes
- Faulty throttle position sensor
- Throttle position sensor harness is open or shorted
- Throttle position sensor circuit poor electrical connection
- Faulty Powertrain Control Module (PCM)

Try rebooting the PCM.

This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed.

After performing this procedure the 1996-2001 PCM will re-learn and store into Adaptive Memory your engines performance characteristics.

Please perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn Mode adaptive mode.

Disconnect the POSITIVE battery Terminal and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”


The PCM Adaptive memory has now been flashed, or erased from the PCM.

When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory.

When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”.

The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.

Well I guess I need to revive this thread...

I had previously fixed this issue by replacing it with ANOTHER tps sensor. I got another one from the dealership and noticed that the part number for the new one was different then the last one they had given me, so I figured they sold me the wrong one the first time since this new sensor fixed all my issues. It's been in my jeep for about a month now without any problems. However, I do not DD my jeep anymore... it saw maybe 500km total in the last month. Driving maybe once a week. So that TPS should be still like brand new almost.

Until now...
I jumped in my jeep this morning to drive to work, and everything was good until about 5 blocks from home, it just went into limp mode again! Same thing as before... it would not allow me to go any faster then 30km/h, it wouldn't shift gears even if I put it in 1-2 or 3 instead of D. When I put it in N it revs up to about 2000rpm. Turn it off/restart did nothing... But I have no engine codes this time.


I am going to try reooting the PCM tonight when I get back home. I sincerely hope that works because this issue is really starting to annoy me.

Is there any way a vacuum leak could cause these issues?

CPS going bad is my first wild guess, or the VSS sensor perhaps, or its wiring. Or the fuel pump loosing pressure (I hear a lot about hoses coming loose on the pump in the gas tanks lately.

If the 98 has a distributor check the rotor and cap closely too.

The throttle position sensor is DIRECTLY involved with transmission shifting characteristics.

The TPS is sensitive to heat, moisture, and vibration, leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following:

-Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".

- High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.

-Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.

-Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted accordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.

- Engine Misfire: A faulty TPS can report values outside the acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessive misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

I had bad luck with a new TPS. A trip to the junk yard yielded a handful of used units, for about five bucks apiece. I tried them one by one until I found one the HEEP was happy with. Tim has pretty much outlined all of the problems I've experienced in my '96 while suffering through various funky TPS's.

Gotta follow the codes, wherever they lead you. IF one comes back and it likely will. You may even have a pending code, even though the check engine light isn't on. The OBDII system is far from perfect, but it is awfully darn good at pointing you to the offending circuit.

If you continue to see codes relating to the TPS, you need to concentrate on the wiring and connector. Have you tried flushing the connector with a thorough spraying of electrical contact cleaner and adding a dab of dielectric grease and reconnecting? I'd sure try that.

I'm just getting some free time now to look into this... Ill start on it tomorrow.

Pulled the codes today though and I got a p0122 and p0123 and a p0700. Dunno where that 700 came from but I'm thinking it's time to clean the NSS.

I wish the junkyards around me had 97+XJ's. I'm lucky if I find one a year in the local yards. They get picked over really fast. A new TPS costs $100 even for the econo line. I'd love to get a handful of used tps's to test out.

http://www.jeepforum.com/forum/f9/error-codes-p0122-p0123-428667/

Those codes all sound like a TPS or TPS wire problem. You can test the input and output with a volt meter!! Time to get in there with a volt meter.

Sounds like that water got into the wires, wiring harness, connector, maybe with soap is shorting some damaged wires out.

pull the tps plug off and tighten the pins that go to it or get a new connector for it, alot of problems are just loose connectors

Jnida63 said:

pull the tps plug off and tighten the pins that go to it or get a new connector for it, alot of problems are just loose connectors

Click to expand...

I know that helps on the older jeeps, like Renix, but his a 98. Do they have the same problem?

I may have got it figured out...

I pulled another tps from the junkyard and when I went to install it, I popped my hood and saw a pile of snow directly on the tps!

It seems the snow/rain goes through my Lebaron vents directly onto the sensor. I never noticed it before, but the time I started having these issues does correspond with the approx. time I put the vents in. DOH!

I built a drip pan from some aluminum and bolted it under the vent and put the junkyard tps in and so far so good after 500km and 3 more snowfalls

Still going to keep my eyes open at the junkyards for more tps's... $10/sensor is a LOT cheaper than what they want at the dealer!

Nice catch. Funny how those things happen, and looking back it seems like an obvious solution.

ert01 said:

I may have got it figured out...

I pulled another tps from the junkyard and when I went to install it, I popped my hood and saw a pile of snow directly on the tps!

It seems the snow/rain goes through my Lebaron vents directly onto the sensor. I never noticed it before, but the time I started having these issues does correspond with the approx. time I put the vents in. DOH!

I built a drip pan from some aluminum and bolted it under the vent and put the junkyard tps in and so far so good after 500km and 3 more snowfalls

Still going to keep my eyes open at the junkyards for more tps's... $10/sensor is a LOT cheaper than what they want at the dealer!

Click to expand...

Check out "tps issues after steam clean". I recently posted what I have found

Left Tim a message, he's quite the guru. Maybe someone else run into this; 2000 cherokee 4.0l 5 sp 135,000mi's. runs smooth as silk, NO other symptoms, except better gas mileage; flattens out at 3 grand, will not go over 4 then codes 0121 BUT tested TPS, everything fine, replaced it to be sure, cleaned throttle body, replaced converter, tested pressure on rest of exhaust system fine. The eng light goes off next start, but if I try to push it, comes back on. Cleared the ECM per instructions several times, still doing the same thing with no other symptoms, runs smooth and no chugs glugs spits backfires, nothing. I have to downshift up steeper grades when I never had to before. I always seem to have the troubles which "you go HHHMMM....."

P0121 definition is "TPS position does not agree with MAP signal."

It's entirely possible for a bad MAP sensor or vacuum leak to cause this code.

Well, I ordered a map sensor, in the mean time my wife with hyper hearing (she can hear me cussing in the shop 50 feet away) said she did hear a whistling. Gonna dig into it with a squirt bottle and vac gauge and see what I find. The new converter wasn't a total waste, it's a lot more open and gave it a throatier growl through the walker muffler.