OBD-II trouble code
P0155: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
The internal heater in the upstream oxygen sensor on bank 2 isn't drawing the current the PCM expects. Either the heater element has failed, the supply or ground wire is broken, or the PCM driver isn't switching the circuit on.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $5 – $500
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0155 mean?
P0155 is the bank-2 mirror of P0135, and to understand why it matters you have to understand what an O2 sensor heater is actually doing. A modern wideband or narrowband O2 sensor needs to reach roughly 600-700°F before it produces an accurate signal. Waiting for exhaust gas heat alone would mean 2-4 minutes of warm-up time on a cold morning. During those minutes the engine runs in open-loop mode — using a richer fuel map than it really needs — because the PCM doesn't trust the sensor yet. Open-loop operation means higher emissions, worse fuel economy, and on a cold engine, more raw fuel reaching the catalyst.
Emissions regulations set limits a vehicle cannot meet if it spends long in open loop, so modern calibrations are built to reach closed loop within roughly the first minute after startup. That is only possible because of the internal sensor heater — a small resistive element that warms the sensor up directly, separate from exhaust gas heat. The PCM monitors the current draw on the heater circuit to confirm it's working. If the current is too high, too low, or absent entirely, P0155 sets.
The failure modes fall into three groups. The heater element failing inside the sensor is the most frequently reported, as a normal age-related failure on higher-mileage vehicles. Wiring problems come next — chafe damage, a broken pin, melted insulation near the exhaust heat. The rest are blown fuses (the heater circuit usually has dedicated fuse protection), occasional PCM driver failures, and corrosion in the connector preventing reliable current flow. Which of the two banks is the awkward one to reach depends on where cylinder 1 sits on your engine, so do not assume bank 2 carries an access premium — on several common transverse V6 families it is the easier side. The diagnostic process is the same either way.
Common causes
- Blown fuse on the O2 sensor heater supply circuit — the cheapest outcome and the one to check first
- Corroded or backed-out pin at the sensor connector
- Damaged wiring on the heater supply or ground side
- Failed heater element inside the bank-2 upstream O2 sensor — the most frequently reported cause on higher-mileage vehicles
- Melted wiring or connector from exhaust manifold heat exposure
- Aftermarket exhaust or header work that damaged the harness
- PCM heater driver failure (rare)
- Recent battery service that left the heater fuse disturbed or seated incorrectly
Symptoms
- Check Engine Light on
- Worse fuel economy on short trips and cold-weather driving (engine stays in open-loop longer)
- Stored freeze-frame data showing the fault set within the first 30-60 seconds after startup
- Failed emissions test — the readiness monitor for the heater system won't complete
- Sometimes a hesitant or rich cold-start condition
- On longer drives, fuel economy returns to normal once exhaust heat warms the sensor (masking the issue between trips)
Diagnostic steps
- 1.Verify the fault is on bank 2, sensor 1 (the upstream sensor on the cylinder bank that does NOT contain cylinder 1) before pulling tools.
- 2.Check the O2 sensor heater fuse. Many platforms run all four sensors on one or two fuses; a single blown fuse can take out multiple sensors at once.
- 3.Disconnect the bank-2 upstream sensor and measure heater terminal resistance. Most spec 4-15 ohms; an open circuit or way-out-of-range reading condemns the sensor.
- 4.Probe the heater supply pin at the harness side of the connector with the engine running rather than key-on-engine-off. On many vehicles that feed comes through a relay which only closes once the engine is turning, so zero volts at rest is normal there and is not evidence of a fault — check the wiring diagram before chasing an open.
- 5.Probe the heater ground/return pin. On the common arrangement the PCM pulses this side, so with the engine running it should switch between voltage and ground; some platforms switch the supply side instead, so confirm which before treating a steady reading as a fault. This sensor sits close to the exhaust manifold — keep hands, leads and sleeves clear of the manifold, the belts and the cooling fan while the engine runs.
- 6.Inspect the connector for melted insulation, oil contamination, or backed-out pins. Bank-2 upstream sensors often live close to the exhaust manifold and see severe heat.
- 7.If heater resistance and wiring all check good, scope the PCM driver signal — a flat-line driver is a module-side issue.
- 8.Swap-test with bank-1 upstream sensor (if same part number) before condemning the sensor for certain.
Repair cost
$5 – $500
Blown fuse fix: under $5. Bank-2 upstream O2 sensor replacement typically $200-450. The spread is access, not the bank number: whichever bank is buried on your engine carries the labour, and on several common transverse V6 families that is bank 1 rather than bank 2. Wiring repair varies widely: $50-500 depending on whether a section of harness needs to be re-loomed. PCM driver failures are rare but expensive when they happen ($600-1,500).
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Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.