09-07-2021
12 min.
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P0155 O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)

P0155 is an OBD-II diagnostic trouble code indicating that the engine control module (ECM/PCM) has detected a problem with the oxygen sensor heater circuit for Bank 2, Sensor 1.

Unlike codes that describe an O2 sensor signal being too high, too low, slow, or inactive, P0155 specifically points toward the heating circuit of the oxygen sensor.

This distinction is important because the oxygen sensor itself may still be capable of producing a signal. The problem may instead be related to the heater's power supply, ground, wiring, fuse, relay, connector, or control circuit.

What Is Bank 2 Sensor 1?

The location of the sensor is part of understanding the code.

Bank 2 is the cylinder bank that does not contain cylinder number 1.

Sensor 1 generally refers to the oxygen sensor located before the catalytic converter, often called the upstream O2 sensor.

Therefore, P0155 concerns the upstream oxygen sensor on Bank 2.

On a V6 or V8 engine, there are normally two cylinder banks. One contains cylinder number 1 and is Bank 1; the opposite bank is Bank 2.

On an inline engine with only one cylinder bank, Bank 2 normally does not exist. However, sensor configurations vary, so the manufacturer's service information should always be used to confirm the sensor location.

Why Does an Oxygen Sensor Need a Heater?

An oxygen sensor needs to reach a specific operating temperature before it can provide accurate information to the ECM.

Older oxygen sensors depended heavily on exhaust heat to reach operating temperature. Modern heated oxygen sensors contain an internal heating element that allows them to become operational much faster.

The heater is particularly important:

After a cold start
At idle
During low engine load
In cold weather
During periods when exhaust temperature is relatively low

By heating the sensing element independently of exhaust temperature, the ECM can begin using the sensor much sooner.

A problem with this heater can therefore affect emissions control and fuel-management strategies.

What Does P0155 Actually Mean?

P0155 does not necessarily mean that the O2 sensor signal itself is incorrect.

Instead, the ECM has detected that the electrical behavior of the heater circuit is outside the expected range.

Depending on the vehicle, the ECM may monitor things such as:

Heater current
Heater resistance
Voltage
Circuit continuity
Heater activation
Electrical response after the heater is commanded on

If the expected response is not detected, P0155 may be stored.

The exact monitoring strategy is manufacturer-dependent.

The Heater Is a Separate Part of the Sensor's Function

An oxygen sensor has two fundamentally different aspects:

The sensing element

This measures oxygen-related exhaust conditions.

The heating element

This brings the sensor to the required operating temperature.

A sensor can therefore have a functioning sensing element while having a failed heater.

Conversely, the heater may operate correctly while the sensing element itself has become slow or inaccurate.

This is why a heater circuit code should not automatically be treated as a general O2 sensor signal failure.

Common Causes of P0155

The oxygen sensor itself is only one possible cause.

Failed O2 sensor heater

The internal heating element can burn out or develop excessive resistance.

In this situation, the sensor may still generate a signal once sufficiently heated by the exhaust, but it may take too long to become operational.

Blown fuse

Some vehicles supply heater power to multiple oxygen sensors through a shared fuse.

If the fuse is blown, the affected sensor heater may not receive power.

A critical diagnostic point is to determine why the fuse failed.

Replacing a blown fuse without checking for a short circuit can result in the new fuse failing again.

Damaged wiring

O2 sensor wiring operates in a very harsh environment.

The harness is exposed to:

High exhaust temperatures
Engine movement
Vibration
Oil contamination
Water
Road debris
Mechanical contact

Heat-damaged insulation is particularly common around exhaust manifolds and catalytic converters.

Connector Problems

A damaged or corroded connector can interrupt the heater circuit.

Inspect for:

Loose terminals
Corrosion
Bent pins
Water intrusion
Burned terminals
Poor terminal tension
Broken locking mechanisms

A connector may look clean from the outside while having insufficient terminal tension internally.

Heater Power Supply Problem

On many vehicles, one side of the heater receives power through a fuse or relay, while the ECM controls the other side.

However, the circuit architecture is not universal.

Depending on the vehicle, the ECM may use different strategies for controlling and monitoring the heater.

The wiring diagram should therefore be consulted before applying power or ground to any O2 sensor terminal.

Ground or Control Circuit Problem

A missing ground or control signal can prevent the heater from operating.

Possible faults include:

Open control wire
Short to ground
Short to power
Poor ECM ground
Damaged harness
Faulty relay
Faulty heater control circuit

The ECM may detect that it commanded the heater but did not observe the expected electrical response.

Symptoms Associated With P0155

Some vehicles show very few noticeable symptoms.

Possible symptoms include:

Check Engine Light
Increased emissions
Reduced fuel economy
Longer time before closed-loop operation
Rougher operation during cold operation
Slight hesitation after startup
Failed emissions testing
Other O2 sensor or fuel-trim codes

Because Sensor 1 is generally an upstream sensor, a heater problem can have a greater effect on engine-management operation than a similar heater fault involving a downstream sensor.

Why Sensor 1 Is More Important for Fuel Control

The upstream oxygen sensor generally provides important feedback for air-fuel mixture control.

After the engine starts, the ECM may initially operate in open loop while conditions stabilize.

Once the O2 sensor reaches operating temperature, the ECM can use its information for closed-loop fuel control.

If the heater is not functioning correctly, the sensor may take longer to reach the required temperature.

This can affect fuel control, especially during cold operation.

The exact strategy differs between vehicles.

How to Diagnose P0155

The best approach is to treat P0155 as an electrical circuit diagnosis, rather than immediately replacing the oxygen sensor.

Start by scanning the vehicle for all codes.

Look for additional faults involving:

O2 sensor heaters
Battery voltage
Fuel mixture
Engine coolant temperature
Other oxygen sensors
ECM power supply
Communication

If several O2 heater codes appear simultaneously, a shared fuse, power supply, relay, or wiring problem becomes particularly important.

Check the Freeze-Frame Data

Freeze-frame information can show the conditions under which the ECM detected the problem.

Useful information includes:

Engine coolant temperature
Engine RPM
Vehicle speed
Battery/system voltage
Engine load
Fuel-system status

If the code occurs immediately after a cold start, the heater circuit deserves particular attention.

Inspect the O2 Sensor and Harness

Before performing electrical measurements, inspect the physical installation.

Follow the wiring from Bank 2 Sensor 1 toward the main harness.

Look for:

Melted insulation
Wiring touching the exhaust
Chafing
Broken wires
Pinched wires
Oil contamination
Loose connectors
Incorrect routing

A wire can be damaged internally even when its insulation appears acceptable.

Check the Fuse

If the oxygen sensor heater receives power through a fuse, verify the fuse according to the vehicle's wiring diagram.

Do not simply install another fuse and assume the problem is solved.

If the replacement fuse blows again, there may be:

A shorted heater
Shorted wiring
A damaged connector
Another component sharing the circuit

The cause of the excessive current should be found before continuing.

Measure Heater Resistance

The internal heater can often be tested by measuring resistance across the appropriate heater terminals.

However, there is no universal O2 heater resistance value.

The correct resistance depends on:

Sensor design
Manufacturer
Engine
Sensor generation
Operating temperature

Use the manufacturer's specification.

A heater that measures open circuit is obviously suspicious, but a resistance measurement within range does not necessarily prove the complete circuit is functioning correctly.

Check Heater Voltage

If the heater circuit has a power supply, verify whether the expected voltage reaches the sensor.

This should be done using the manufacturer's test procedure.

Check:

Power supply
Ground/control side
Voltage drop
Connector condition
Fuse/relay operation

Testing only for voltage with the circuit disconnected may not reveal a high-resistance connection.

Voltage Drop Testing Is Valuable

A circuit can show normal voltage with little or no load but experience a significant voltage drop when the heater is operating.

This is why voltage-drop testing can be more informative than simply checking continuity.

A high-resistance:

Connector
Terminal
Wire
Ground connection
Fuse connection

can prevent the heater from receiving enough effective power even though a basic voltage test looks acceptable.

Check the Heater Current

Where the manufacturer's diagnostic procedure permits it, heater current can provide valuable information.

A heater that draws no current may indicate:

Open heater element
Open power circuit
Open ground/control circuit

Excessive current may suggest:

Shorted heater
Wiring problem
Internal sensor fault

Again, the expected current depends on the specific vehicle and sensor.

Do Not Apply Battery Voltage Randomly

O2 sensors are not all wired the same way.

Do not assume that two similarly shaped wires or terminals can be connected directly to battery power.

Incorrectly applying battery voltage to a signal, reference, or control circuit can damage the sensor or, in some systems, the ECM.

Always identify the heater terminals using the wiring diagram.

Could Low Battery Voltage Cause P0155?

Low system voltage can affect heater operation, particularly during starting.

If P0155 appears together with:

Battery-related codes
Multiple communication codes
Slow cranking
Charging-system problems
Other electrical faults

the battery and charging system should be checked.

However, a normal battery does not eliminate an O2 heater circuit fault.

Check system voltage under the conditions specified by the manufacturer.

Could the ECM Cause P0155?

Yes, but the ECM should generally be considered after the external circuit has been verified.

Before condemning the ECM, confirm:

Correct heater power
Correct ground/control
Fuse and relay operation
Wiring integrity
Connector condition
Heater resistance
Voltage drop
Current draw

A damaged ECM driver is possible, but it is considerably easier and more common to find a wiring, connector, fuse, or sensor problem.

What About the O2 Sensor Signal?

Because P0155 is a heater code, the sensor signal should not be the first thing to diagnose.

However, once the heater circuit is repaired, sensor operation should be verified.

Check whether Bank 2 Sensor 1 reaches operating temperature and begins providing appropriate data.

If additional codes such as slow response, low voltage, high voltage, or range/performance faults are present, the sensor itself and engine operating conditions may also need investigation.

Compare Bank 1 Sensor 1 With Bank 2 Sensor 1

If the engine has two cylinder banks, comparison can be extremely useful.

Observe:

Bank 1 Sensor 1

versus

Bank 2 Sensor 1

Compare their behavior during:

Cold start
Warm-up
Closed-loop transition
Idle
Cruise

If Bank 1 Sensor 1 becomes active normally while Bank 2 Sensor 1 remains delayed, the Bank 2 heater circuit becomes a stronger suspect.

This comparison is particularly useful when the vehicle stores only P0155 without obvious drivability symptoms.

Multiple O2 Heater Codes Are an Important Clue

Suppose a vehicle reports heater faults for several oxygen sensors simultaneously.

Replacing all the sensors would be a poor first step.

A shared problem could be responsible, such as:

Common fuse
Shared power supply
Relay
Harness damage
Common ground
Low system voltage

The wiring diagram should be checked to determine which parts of the heater circuit are shared.

Can P0155 Be Caused by a Lean or Rich Mixture?

A lean or rich engine condition does not normally create the basic electrical heater fault represented by P0155.

However, engine problems can sometimes create additional O2-related codes at the same time.

For example, an engine with a fuel-system problem may produce:

Fuel-trim codes
O2 signal codes
Misfire codes

alongside an independent heater circuit problem.

The complete DTC list should therefore be considered rather than assuming every code has the same cause.

Is P0155 the Same as P0135?

These codes have the same basic heater-fault concept but refer to different sensor locations.

P0135 generally means:

O2 Sensor Heater Circuit Malfunction – Bank 1 Sensor 1

P0155 generally means:

O2 Sensor Heater Circuit Malfunction – Bank 2 Sensor 1

The important difference is the cylinder bank.

Bank 1 contains cylinder number 1, while Bank 2 is the opposite bank.

Is P0155 the Same as P0161?

Both are oxygen sensor heater circuit codes, but they concern different sensors.

P0155 → Bank 2 Sensor 1

P0161 → Bank 2 Sensor 2

Sensor 1 is generally upstream of the catalytic converter, while Sensor 2 is generally downstream.

A More Efficient Diagnostic Strategy

Instead of immediately replacing the sensor, use this sequence:

Scan all stored and pending DTCs.
Record the freeze-frame data.
Confirm the location of Bank 2 Sensor 1.
Inspect the sensor and harness.
Check the connector and terminal condition.
Check the heater fuse and relay if equipped.
Identify the heater circuit using the wiring diagram.
Measure heater resistance according to specification.
Verify heater power and control/ground.
Perform voltage-drop testing where appropriate.
Check heater current if specified.
Compare Bank 2 Sensor 1 with Bank 1 Sensor 1.
Check battery and charging-system voltage if other electrical faults are present.
Repair the wiring, power supply, connector, or sensor according to the test results.
Clear the DTC.
Bring the engine through the required operating conditions.
Verify that the O2 heater monitor completes without P0155 returning.

When Should the O2 Sensor Be Replaced?

Replacing the sensor makes sense when testing shows that the internal heater is defective or outside the manufacturer's specification.

For example, if:

The heater element is open
Heater resistance is outside specification
Correct power and control are present
Wiring and connector are good
There is no excessive voltage drop

then the O2 sensor's internal heater becomes a strong suspect.

But if the heater has no power because of a broken wire or blown fuse, replacing the sensor will not solve the problem.

Is It Safe to Drive With P0155?

A vehicle with P0155 will often remain drivable, particularly if the engine runs normally and the Check Engine Light is steady.

However, the fault should not be ignored.

Because Bank 2 Sensor 1 is generally an upstream sensor, prolonged heater problems can affect emissions control and cold-start fuel management.

Driving should be minimized if the code is accompanied by:

Flashing Check Engine Light
Severe misfire
Significant loss of power
Stalling
Strong fuel odor
Severe engine hesitation

Those symptoms may indicate a separate or additional problem that is more urgent than the heater fault itself.

Final Diagnosis

P0155 – O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1) identifies a problem with the electrical heating circuit of the upstream oxygen sensor on Bank 2.

The fault may be caused by the O2 sensor heater element, fuse, relay, wiring, connector, power supply, ground/control circuit, or, less commonly, the ECM.

The most important diagnostic mistake to avoid is assuming that P0155 automatically means the oxygen sensor must be replaced.

The correct approach is to verify the heater circuit electrically, check power and control under the proper conditions, inspect the harness around the hot exhaust system, and confirm the sensor's heater resistance and current behavior against manufacturer specifications.

In short, P0155 is primarily an O2 heater circuit diagnosis, not a generic oxygen-sensor signal fault.