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1998 Jeep Wrangler 4.0L P0301 Cylinder 1 Misfire & P0123 Code - Diagnosis and Fixes

Model: 98 Jeep Wrangler Fault Code: P0123 Posted: 2018-01-29 13:09

I own a 1998 Jeep TJ with a 4.0L engine and have been experiencing a persistent P0301 code indicating a cylinder 1 misfire for over four years. I've already replaced spark plugs, ignition wires, distributor cap, rotor, oxygen sensors, and the throttle position sensor (TPS). I've also swapped injectors between cylinders and performed a leak-down test across all cylinders. Additionally, I’ve tried disconnecting and reconnecting the powertrain control module (PCM), but the P0301 code returns every time after clearing it. Recently, the vehicle has started showing a new fault: P0123. The car now struggles to start and, once running, exhibits significant throttle fluctuation. Despite extensive troubleshooting, the issue remains unresolved. I'm seeking reliable diagnostic steps and potential root causes that could explain both codes in this 1998 Jeep Wrangler. Any insights or suggestions on how to properly diagnose and resolve these issues would be greatly appreciated.

Related fault codes

P0123P0301

Comments (2)

Anonymous 2018-01-29 13:19

This appears to be a 4.0L engine. For P0301 (cylinder 1 misfire), it's important to verify the spark plug brand and type — not all plugs perform equally in older Jeep engines. Check for vacuum leaks, especially at the intake manifold gaskets or connections. A simple spray test with carburetor cleaner can help identify such leaks. Have you performed a compression test? If not, it's recommended to do so to rule out mechanical issues in cylinder 1. For P0123 (TPS signal voltage issue), measure the TPS output voltage under different engine conditions — particularly at idle and full throttle. A drop or inconsistency may indicate faulty sensor wiring or a failing TPS. Are there any other symptoms? For example, cruise control malfunction, horn issues, or airbag warning light? Do you have access to an OBD2 scanner? A device like the Torque app (via Bluetooth) can help read live data and fault codes in real time.

Anonymous 2018-01-29 15:26

I don’t remember the exact brand or type of spark plugs I used, but it’s been about four years since I began dealing with this issue. I did perform a compression test and a vacuum leak spray test previously — though repeating these might be worthwhile. I’m not familiar with how to conduct a fuel pressure test, so I’ll look into the proper procedure. I also haven’t measured TPS voltage yet — I plan to check that soon. No other noticeable issues beyond the starting difficulty and throttle instability. There is an aftermarket cruise control module installed, but it doesn’t function properly. I do have a Bluetooth OBD2 scanner and use the Torque app for live data monitoring. This allows me to track real-time sensor readings when needed.