P0138 Fault on 2000 Jeep TJ: Causes and Solutions for O2 Sensor Circuit Issues

The most likely cause is a faulty oxygen (O2) sensor or an issue with its wiring and circuit. P0138 specifically indicates a high voltage in the O2 sensor circuit for Bank 1, Sensor 2 — which is located after the catalytic converter on the driver's side.

How can I determine which O2 sensor is affected? Is there a way to identify whether it’s the upstream or downstream sensor based on location or wiring?

P0138 – O2 Circuit High Voltage (Bank 1, Sensor 2). This refers to the downstream oxygen sensor after the catalytic converter on Bank 1. While it may not directly cause a rich condition, a malfunctioning sensor can lead to incorrect air-fuel ratio readings and potentially result in running too rich.

I agree — that's a strong suspect. Does the vehicle show any signs of misfiring? What is the engine size and configuration?

It’s a 4.0L inline-6 engine with no misfires. The truck runs well under all conditions, including idle and highway driving.

Start by testing the O2 sensor itself. Also inspect the wiring harness and check for any fuses labeled 'O2 sensor' in the fuse box. A blown fuse could prevent proper signal transmission — even if the sensor is new, a missing power supply will cause erratic readings.

I have the same issue. I replaced the O2 sensor behind the catalytic converter, but after about 3 miles of driving, the engine cuts off. After a 3- to 5-second delay, it restarts and runs for another 2 miles before cutting out again. This cycle repeats two to three times before the vehicle stops completely. I’ve had to wait several hours before restarting — any advice? I'm very frustrated. Vehicle: 2000 Jeep TJ

Check fuel pressure and exhaust backpressure. Also, verify when the last time the fuel filter was replaced. These factors can influence O2 sensor performance and contribute to incorrect air-fuel mixture readings.

Exhaust backpressure should be checked first. Additionally, ensure that the O2 sensor circuit is functioning properly — many aftermarket sensors fail to heat up adequately, which can cause inconsistent readings and trigger P0138 even after replacement.

I own a 2001 Jeep Wrangler TJ with 136K miles. I recently replaced the downstream O2 sensor (after the catalytic converter), and the vehicle has improved noticeably — more power, smoother operation. The check engine light remains on with P0138 and a pending code that clears but returns. I’ve been told that some models have fuel filters located inside the fuel pump assembly, and they shouldn’t be replaced unless there’s fuel contamination. Given this mileage, I’m considering replacing the fuel filter as a preventive step. Any recommendations? Vehicle: 2001 Jeep Wrangler TJ (same platform as 2000 TJ)

A fuel filter replacement will not resolve P0138 if the issue is with the O2 sensor or its circuit. If the fuel filter is located inside the fuel pump (as in some older Jeeps), replacing it without contamination issues may be a waste of time — unless you're doing it as part of a full fuel system overhaul. What brand of O2 sensor did you install? How many sensors are present on your Jeep? (e.g., 2, 3, or 4?) Are you comfortable using a digital volt-ohmmeter (DVOM) and reading wiring diagrams to test the O2 circuits? Is there any blown fuse labeled 'O2 sensor' in the power distribution panel?

Answers: - Bosch O2 sensors, both upstream and downstream. - Total of two sensors: one after the exhaust manifold (upstream), one after the catalytic converter (downstream). - There is a single fuse for both sensors — it shows a short circuit when tested. - A relay labeled 'O2 Sensor' exists in the power distribution center. It shares the same part number as the horn relay. I swapped it with the horn relay, and the horn still works — but the check engine light remains on. - I can use a DVOM and have access to a wiring diagram (attached). However, I don’t know what the expected voltage should be at pin 4 of the downstream sensor. - Note: Earlier today, before reseating fuses or relays, the check engine light was on but no code was present. After resetting, the light came back on after a few minutes of driving — this occurred four times throughout the day with consistent results (light on, no stored code).

Bosch sensors? I’d recommend returning them and replacing with OEM or dealer-grade sensors — aftermarket units often don’t heat up properly and fail to meet required voltage thresholds for proper operation.

This post appears to be a dead end — no real value. Why was it brought up? Was this meant to promote parts sales? It seems off-topic and unhelpful.