What it means
Same condition as P0135 but on the upstream sensor of bank 2. The sensor heater warms the element to operating temperature (~600 °C); without it the lambda sensor stays inactive in closed loop and the ECM cannot trim mixture.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 2 sensor 1 is the upstream oxygen sensor, and its heater brings the sensing element up to operating temperature after startup. P0155 sets when the ECU detects a fault in that sensor’s heater circuit, which can delay reliable closed-loop fuel control while the exhaust is cold.
Common causes
Most likely first.
- Oxygen sensor heater element failed
- Heater circuit fuse or relay fault
- Heater wiring or connector damage
- Poor electrical ground or power supply
Symptoms
- Check engine light on after several drive cycles
- Slightly increased fuel consumption during cold start
- Delayed entry into closed-loop fuel control
- Failed emissions inspection (lambda readiness)
How to diagnose it
- Use freeze-frame data to note whether the fault occurred during a cold start, then check live data for bank 2 sensor 1 heater command or status if available.
- With the engine off and cool, inspect the upstream bank 2 sensor connector and nearby wiring for heat damage, corrosion, loose pins, or chafing.
- Check the heater circuit fuse and relay or shared power feed using the vehicle wiring diagram, since a supply fault can affect the heater without damaging the sensor.
- Measure heater power, ground, and element continuity at bank 2 sensor 1 and compare results with the manufacturer’s specification.
- If wiring and supply are sound, confirm the heater element is faulty before replacing the upstream bank 2 oxygen sensor.
Common fixes
- Reconnect or clean the bank 2 sensor 1 heater connector.
- Repair damaged heater wiring or a poor ground.
- Replace a failed heater fuse or repair the relay or power-feed fault after checking for a short.
- Replace bank 2 sensor 1 if its heater element fails testing.
Can I keep driving?
You can usually drive gently with a steady light, but cold-start emissions and fuel use may rise until the heater works. Avoid prolonged driving if the engine runs poorly or the warning light flashes.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.