Each oxygen sensor has a small heater that brings it to working temperature soon after a cold start. On V-type engines, the front sensor on bank 2 often sits in a cramped, hot spot that is hard on its wiring. A P0155 code means the engine computer found a fault in the heater circuit of the bank 2 upstream oxygen sensor.
This guide explains how the heater works, how to test it and which repairs fix the code.
P0155 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 55 | Specific fault | O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1) |
What the P0155 Code Means
The SAE J2012 standard defines the P0155 code as O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1). It points to the heater of the front oxygen sensor on the side of the engine without cylinder 1.
A zirconia sensor must reach about 600°F before it produces a usable signal. The heater gets it there within a minute or two, so the engine enters closed loop sooner. Most heaters receive battery voltage through a fuse, and the powertrain control module (PCM) switches the ground side.
The PCM monitors heater current or the control wire voltage. If the current is too low, too high or missing, it sets the code.
Symptoms of a Bank 2 Front Heater Fault
Most drivers see only the warning light. Common signs of a P0155 code include:
- Check engine light on
- Longer open loop time after a cold start
- Slightly lower fuel economy on short trips
- Higher emissions after startup
- No activity code P0154 stored too
Common Causes of the P0155 Code
The heater element itself fails most often. These are the usual causes:
- Burned-out heater element. Heat cycles wear out the heater over time.
- Blown fuse. A shorted heater or wire can blow it.
- Heat-damaged wiring. Wires near the bank 2 manifold can melt or break.
- Corroded or burned connector. High resistance at the pins lowers heater current.
- Failed heater relay. Some vehicles power heaters through a relay.
- PCM driver fault. Rare, but possible.
How to Test the Heater Circuit
You need a digital multimeter, a test light and the wiring diagram. Let the exhaust cool before working near the sensor.
Check the Fuse
Find the oxygen sensor heater fuse in the wiring diagram or fuse box legend. Test it. If it is blown, replace it once. If it blows again, there is a short in a heater or its wiring.
Measure Heater Resistance
Unplug the bank 2 front sensor and find the two heater pins. Measure resistance across them on the sensor side. Most heaters read about 2 to 30 ohms at room temperature. Compare the reading with the spec for your sensor. An open reading means the heater has failed.
Check Power and Ground
With the key on and the sensor unplugged, measure the heater power wire on the vehicle side. It should read close to battery voltage. Connect a test light between the power and control wires and start the engine. The light should come on when the PCM turns the heater on.
How to Fix the P0155 Code
Match the repair to your tests. Then clear the code and drive through a cold start so the PCM can retest the heater.
- Replace the bank 2 upstream sensor if the heater reads open or out of spec.
- Replace a blown fuse after fixing the short.
- Repair burned wiring and secure it away from the manifold.
- Clean or replace the connector if the pins are corroded or melted.
- Replace the heater relay on vehicles that use one.
If the heater fuse keeps blowing, unplug the oxygen sensors on that fuse one at a time. When the fuse stops blowing, the last sensor you unplugged has a shorted heater. This simple test keeps you from replacing good sensors.
Repair Cost Breakdown
A new sensor fixes most P0155 code cases. Labor depends on access to bank 2. This table shows typical costs.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Fuse replacement | $1 to $5 | $25 to $75 |
| Wiring or connector repair | $10 to $40 | $100 to $300 |
| Bank 2 upstream O2 sensor | $40 to $150 | $150 to $450 |
| Heater relay replacement | $10 to $40 | $75 to $150 |
Can You Drive with a P0155 Code?
Yes. The sensor still heats up from exhaust gas, only more slowly. Bank 2 stays in open loop longer after each cold start.
That wastes a little fuel and raises emissions, mostly on short trips. The vehicle will also fail an emissions test with the light on. Plan the repair, but it is not urgent.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0152 | O2 Sensor Circuit High Voltage (Bank 2 Sensor 1) |
| P0153 | O2 Sensor Circuit Slow Response (Bank 2 Sensor 1) |
| P0154 | O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 1) |
| P0156 | O2 Sensor Circuit Malfunction (Bank 2 Sensor 2) |
| P0157 | O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2) |
Fixing the Bank 2 Front Sensor Heater
A P0155 code means the PCM found a fault in the bank 2 upstream oxygen sensor heater circuit. A burned-out heater element causes most cases.
- Check the heater fuse first.
- Measure heater resistance at the sensor.
- Test power and control at the connector.
- Replace the sensor if the heater is open.
Frequently Asked Questions
Can I Replace Only the Heater?
No. The heater is built into the oxygen sensor and cannot be replaced by itself. A failed heater means a new sensor. Buy one with the correct connector, wire length and heater rating for bank 2 on your engine.
Why Is the Bank 2 Sensor So Hard to Reach?
On many front-wheel-drive V6 engines, bank 2 sits against the firewall. The front sensor may be hidden behind the engine. A slotted oxygen sensor socket, a long extension and patience help. Some people reach it from below with the vehicle raised safely.
Why Did My New Sensor Set the Code Again?
A wrong-spec sensor, a blown fuse or heat-damaged wiring can bring the code back. Some universal sensors have different heater resistance, which the PCM may reject. Check the fuse and wiring, and use a direct-fit part.
What If Both Front Heater Codes Appear?
If P0135 and the P0155 code appear together, check the shared heater fuse and relay first. Both heaters rarely burn out on the same day. A single blown fuse or bad relay can take out both at once.
Will a Bad Heater Damage My Engine?
No. A failed heater only slows the sensor’s warm-up. The engine uses a little more fuel after cold starts, and emissions rise for a few minutes. The bigger issue is the lit check engine light, which can hide a new problem until you fix this one.