P0155 Code: O2 Sensor Heater Circuit Malfunction — Bank 2 Sensor 1
Quick Answer
The heater element in the upstream oxygen sensor on the passenger side isn't functioning correctly.
At a Glance
What this code means
P0155 is the Bank 2 version of P0135. It affects the upstream oxygen sensor before the catalytic converter on the passenger-side bank (Bank 2 on V6/V8 engines). The heater circuit keeps the sensor at operating temperature during warm-up for accurate fuel mixture readings.
Sensor 1 (upstream) measures air/fuel mixture before the cat. Sensor 2 (downstream) monitors catalyst efficiency after it.
Bank 1 contains cylinder #1. Bank 2 is the opposite bank. Inline 4-cylinder engines only have Bank 1.
Common causes
- Failed O2 sensor heater element
- Damaged or corroded wiring or connector
- Blown heater circuit fuse
- Short to ground or open circuit in the heater wiring
Symptoms you might notice
- Check engine light on
- Slightly increased fuel consumption on short trips
- Possible fuel trim codes appearing alongside P0155
- No major driveability issues in most cases
Can you still drive?
Yes. This is not a driving emergency, but address it to keep the fuel system running efficiently.
How serious is this code?
Low to moderate. The sensor may function adequately once warmed by exhaust heat, but fuel efficiency will suffer slightly during cold starts.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Check for related codes — a companion code may point to the true root cause
- Inspect wiring, connectors, and grounds around the affected sensor or component
- Look for obvious physical damage: cracks, disconnected hoses, or corrosion
- Clear the code, drive the vehicle, and see if it returns — intermittent codes can come from loose connections
- Verify the part is actually faulty by testing it before replacing it
How to troubleshoot it
- Check the O2 sensor heater fuse
- Inspect the Bank 2 upstream O2 sensor wiring harness for damage
- Measure heater element resistance and compare to factory specification
- Replace the Bank 2 upstream O2 sensor if the heater has failed
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- Multimeter
- Fuse tester
- O2 sensor socket
Estimated repair cost range
Upstream Bank 2 O2 sensor replacement: $100–$300.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
On V8 engines, there are four oxygen sensors in total. P0135 affects front-left, P0155 affects front-right. If you're replacing one, it's worth inspecting all sensors, since they tend to wear at similar rates on high-mileage engines.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.