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99 Dodge Ram 1500 5.9L Z Engine P0123 Code: Diagnosis and Fix for 4x4 Vehicle with Idle and Acceleration Issues

Model: 99 dodge ram 1500 Fault Code: P0123 Posted: 2010-06-24 07:04

I own a 1999 Dodge Ram 1500 4x4 equipped with the 5.9L Z engine, and I'm currently experiencing a persistent P0123 trouble code — which indicates a 'Throttle Position Sensor (TPS) Voltage Too High' fault. After extensive troubleshooting, I've confirmed that the issue is not due to broken wires: continuity tests on both power (pin 17) and signal (pin 23) wires show no breaks at either end of the TPS harness. The ground connection also checks out with continuity at both the battery and PCM ground strap. I tested the power supply wire when the engine was off and measured 5.2V — this is below the expected voltage, but I'm unsure if it's significant. I replaced the TPS unit with a new one (Borg Warner), but the P0123 code returned immediately after clearing. My OBD-II scanner shows that once cleared, the code reappears within seconds of starting the engine. The only illuminated warning light is the Check Engine Light (CEL). The ABS light does not stay on. The truck runs smoothly at idle — idling evenly and steadily for extended periods — but during acceleration, it exhibits a noticeable lag. I hear a vacuum intake sound as if the engine is struggling to draw air, followed by a sudden catch-up in power. When I release the throttle, the engine often chokes or stalls. I've verified that my battery is fully charged and all ground connections are clean. I'm unable to take it to a mechanic due to time and cost constraints. This vehicle serves as both my primary transportation and work vehicle, so resolving this issue is critical. Given these symptoms and the persistent P0123 code, I suspect either a faulty TPS connector, internal sensor failure, or an underlying map sensor short — especially since the engine behavior during acceleration matches known issues with malfunctioning MAP sensors on 5.9L Dodge engines. I've also not had any vacuum leaks checked or the idle screw adjusted on the throttle body. I'm seeking advice on how to properly diagnose and fix this issue, particularly whether a TPS connector replacement or map sensor inspection is warranted.

Related fault codes
P0123
Comments (7)
Anonymous 2010-06-24 08:55

Try disconnecting the clock spring at the steering column. This can sometimes reset electrical signals and resolve intermittent TPS-related codes in older Dodge trucks.

Anonymous 2010-06-24 09:40

I'm not sure which connector to remove on my steering column. I've removed the shroud but didn't disconnect anything due to airbag safety concerns. Which one should I target — the red-colored plug at the bottom or the large blue one? Should I disconnect the battery first, and how long does it take for the airbag capacitor to discharge?

Anonymous 2010-06-24 13:27

Disconnect the negative battery cable and wait at least two minutes. This discharges the airbag capacitor safely. Look for yellow-colored connectors on the clock spring — these are typically located at the bottom of the steering column. For reference, visit http://www.e-webcatalog.com/wells/ecatalog.aspx to find a compatible clockspring for your 1999 Dodge Ram 1500.

Anonymous 2010-06-24 16:55

I tried disconnecting the clock spring wires, but there was no change in the P0123 code or engine performance. I installed a new TPS unit, and now I'm getting both P0121 (TPS signal too low) and P0122 (TPS signal too high) codes instead.

Anonymous 2010-06-24 18:02

Did the code or performance change after disconnecting the clock spring? What brand of TPS did you use? I recommend replacing the TPS connector — these are known to fail due to corrosion, poor pin fitment, or brittle insulation. Also, check the MAP sensor reading with a scan tool, as a shorted MAP sensor can trigger TPS-related codes on many Dodge models. Have you checked for vacuum leaks or had the idle screw on the throttle body adjusted?

Anonymous 2010-06-24 20:26

The TPS was a Borg Warner model — I learned a hard lesson. I ended up getting two defective units from that brand. Now, after replacing it with an original Dodge OEM unit, the P0123 code is gone for now. I'm also replacing the TPS connector due to visible corrosion at the plug ends — I cleaned them with a small wire brush. I’d rather be safe than sorry. Thanks for the help — this issue has been frustrating me for 3–4 days.

Anonymous 2010-06-25 04:58

Great to hear it's now working! Many users have encountered similar issues with aftermarket TPS parts — not all are reliable. Always consider OEM components when in doubt. I've seen dozens of TPS connectors with pin misalignment, corrosion, or degraded insulation. The dealer should be able to order the correct OEM connector for your truck if needed.