P0155: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
Updated November 14, 2026
Oxygen sensors have a built-in heater so they start working soon after a cold start. P0155 means the engine computer found a problem in the heater circuit of the upstream sensor on bank 2, the side of a V6 or V8 engine without cylinder 1. Until the sensor warms up from exhaust heat, the engine runs without its feedback.
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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- A burned-out heater inside the oxygen or air-fuel ratio sensor (the most common cause)
- A blown fuse or bad relay feeding the heater circuit
- Damaged wiring or a loose or corroded connector near the exhaust
- An engine computer fault (less common)
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light
- Sometimes slightly worse fuel economy or rough running after a cold start
- A failed emissions test or readiness monitors that won't complete
Is it safe to drive?
Usually safe to drive with a steady light, but get it fixed: emissions and fuel economy can suffer, especially on short trips. If the light ever flashes, stop driving and get it checked.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- Check the fuse for the O2 sensor heaters; if it's blown, look for a shorted heater or wire before replacing it.
- With the engine off, unplug the bank 2 sensor 1 connector and measure the heater resistance across the heater terminals, comparing it with your vehicle's spec.
- Know which type you have: many newer cars use a wideband air-fuel ratio sensor upstream, with different heater specs and wiring from a regular narrowband O2 sensor. Buy the type your engine uses.
- Inspect the wiring for melted insulation or contact with the exhaust, and let the exhaust cool before working near it.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
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Questions
What does P0155 mean?
P0155 is the generic OBD2 code for "O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)." Oxygen sensors have a built-in heater so they start working soon after a cold start. P0155 means the engine computer found a problem in the heater circuit of the upstream sensor on bank 2, the side of a V6 or V8 engine without cylinder 1. Until the sensor warms up from exhaust heat, the engine runs without its feedback.
Is it safe to drive with P0155?
Usually safe to drive with a steady light, but get it fixed: emissions and fuel economy can suffer, especially on short trips. If the light ever flashes, stop driving and get it checked.
How do I check P0155 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Check the fuse for the O2 sensor heaters; if it's blown, look for a shorted heater or wire before replacing it. With the engine off, unplug the bank 2 sensor 1 connector and measure the heater resistance across the heater terminals, comparing it with your vehicle's spec. Know which type you have: many newer cars use a wideband air-fuel ratio sensor upstream, with different heater specs and wiring from a regular narrowband O2 sensor. Buy the type your engine uses. Inspect the wiring for melted insulation or contact with the exhaust, and let the exhaust cool before working near it.
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