Jeep Fault Code
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Lean misfire; Hissing sound; Hesitation on acceleration.
System
Not specified
Symptom
Lean misfire; Hissing sound; Hesitation on acceleration.
Diagnosis & fix steps
Oxygen Sensor Indicates Lean.
LOGIC: The O2 sensor is stuck "low," and the PCM has maxed out its fuel compensation.
ACTION: 1. Check for intake manifold vacuum leaks.
2. Inspect for exhaust leaks at the "cracked" manifold (very common on 4.0L).
FIX: Replace the cracked Exhaust Manifold; repair vacuum leaks.
Red Seal field insights
The 4.0L factory exhaust manifold almost always cracks at the "collector" (where the pipes meet). This lets fresh air in, "tricking" the O2 sensor into thinking the engine is lean, which causes the PCM to dump too much fuel.
Same code keeps coming back?
That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.
Read before you touch anything
- This is general reference information, not a repair manual. It is for trained, qualified technicians.
- Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
- Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
- You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
- Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.
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