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1999 GMC Safari
Diagnosis
1999 GMC Safari P0155 Code Returns After O2 Sensor Replacement
March 3, 2026
V
Visitor Original question
The engine was kicking a p0155 code, I replaced the o2 sensor, and cleared the codes A few hundred miles later the code came back, I cleared it again, and the code came back. The eng1 fuse is good. Not sure about next steps.
Wizuto AI Response
The P0155 code on your 1999 GMC Safari points to a heater circuit malfunction in the bank 2 oxygen sensor. Since you replaced the sensor and the code returned, the most likely cause now is a wiring or connector issue in the sensor heater circuit. The fuse being good rules out a blown fuse, but wiring damage, corrosion, or a bad ground are common culprits.
Most likely culprits
- O2 sensor wiring harness: Heat, vibration, or corrosion can break wires or cause poor connections in the heater circuit.
- Poor ground connection: A bad engine ground or sensor ground can cause heater circuit failure.
- Faulty replacement sensor: Rare but possible if the new sensor is defective or not OEM quality.
How to narrow it down
- Visually inspect the O2 sensor wiring and connector for damage, corrosion, or loose pins. Wiggle the harness while monitoring for intermittent faults.
- Check continuity and resistance of the heater circuit wires from the sensor connector back to the fuse box or ECM connector using a multimeter. Heater resistance should be around 3-10 ohms typically.
- Verify the engine ground straps and sensor ground points are clean, tight, and corrosion-free.
- Swap the sensor with a known good OEM sensor if wiring checks out to rule out a bad replacement.
- Scan for any other related codes that might indicate ECM or wiring issues.
Possible parts
- O2 sensor wiring harness (only if damaged or corroded)
- Engine ground strap (only if found loose or corroded)
- Replacement heated oxygen sensor (only if new sensor is faulty)
A few questions
- Which bank is the sensor on? (Bank 2 is passenger side on GM V6s)
- Was the replacement sensor OEM or aftermarket?
- Did you check wiring continuity and heater resistance before replacing the sensor?