P0155 Code: O2 Heater Circuit, Bank 2 Sensor 1 — Causes & Fix

P0155 means the engine computer has found a fault in the heater circuit of the upstream oxygen sensor on Bank 2, the sensor ahead of the catalytic converter on the side of the engine that does not contain cylinder 1. The heater is a small electric element inside the sensor that brings it up to working temperature quickly after a cold start. On its own, P0155 usually means that one sensor’s heater has burned out. Stored alongside P0135, the same fault on Bank 1, it usually means something else: the fuse or power feed the heaters share has failed. And the short that blew that fuse can be on the other side of the engine. That one distinction decides whether a new sensor fixes the car, and this article covers it, along with the six causes in order of likelihood, the tests that separate them and what each repair typically costs.

The table is the short version. Read the triage note under it before driving.

Cause	Telltale Sign	Typical Cost
Burned-out heater in the sensor	P0155 alone; heater pins read open on a meter	$150 to $500
Blown fuse or failed relay	P0155 with P0135 or other heater codes; no power at the plug	$5 to $150
Harness touching the exhaust	Melted or chafed wiring; the fuse blows again after replacing it	$50 to $400
Connector fault	Burnt, corroded or loose terminals at the sensor plug	$20 to $150
Wrong or poor-quality sensor	Code returns soon after a sensor was fitted	$150 to $500
Computer heater driver or software	Sensor, fuse and wiring all test good	$0 to $1,000+

Stop and find the cause before driving far if a fuse has blown, especially if it blows again after you replace it. That means a wire is shorted, often against hot exhaust, and the same fuse may feed other parts of the emissions system. Also stop if the check engine light is flashing, which means misfires. With P0155 alone and a steady light, the engine runs normally once warm and the car is safe to drive while you arrange the tests below.

What the P0155 Code Actually Means

An oxygen sensor only produces a useful signal when it is hot, several hundred degrees, and exhaust gas alone takes minutes to heat it after a cold start. Until the upstream sensors are working, the engine computer runs the engine on preset fuelling, which is less precise and produces more emissions. So the sensors have built-in electric heaters, and the computer switches them on at start-up to bring the sensors into use within seconds rather than minutes.

The computer monitors each heater circuit. Depending on the manufacturer, it watches the voltage on the wire it switches, the current the heater draws, or how quickly the sensor starts responding. If a heater draws no current, draws too much, or the sensor takes too long to come into use, it sets a code. P0155 is the generic code for the Bank 2 Sensor 1 heater circuit. GM calls it “HO2S Heater Performance Bank 2 Sensor 1”, the wording in its bulletin 24-NA-042. The heater check usually runs early in a cold start, so the code tends to set in the first few minutes of a drive.

It helps to be clear about what P0155 does not mean. The heater has its own two wires, separate from the wires that carry the sensor’s oxygen reading. P0155 says nothing about whether the sensor reads oxygen correctly. A sensor can have a dead heater and an accurate signal once the exhaust warms it, or a working heater and a lazy signal, which sets a different code.

Finding Bank 2 Sensor 1

Bank 2 only exists on engines with two exhaust sides, usually V6 and V8 engines, flat engines and some inline sixes with a separate upstream sensor for each half of the engine. It is the side without cylinder 1. Which side that is depends on the engine, so check its cylinder numbering in the service information rather than assuming left, right, front or rear. Sensor 1 is the upstream sensor, mounted in the exhaust manifold or the pipe just after it, before the catalytic converter.

If your engine has only one bank, a four-cylinder for example, P0155 should not normally appear. Re-read the code, and check that the scan tool has the right vehicle selected. On transverse V6 engines, one bank’s upstream sensor is often tucked against the bulkhead and hard to see, so trace the sensor’s wire back to its own connector before unplugging anything. Plugging a test into the wrong sensor is an easy way to replace a good part.

Symptoms You May Notice Alongside the Code

• The check engine light, often with no change in how the car drives
• Slightly worse fuel economy, mostly on short trips where the engine spends more of its time cold
• A slightly rough or rich-smelling cold start on some engines
• Other oxygen sensor or heater codes, if a shared fuse or wire is at fault
• An emissions inspection failure, because the light is on

1. A Burned-Out Heater in the Sensor

This is the most common cause when P0155 is the only heater code. The heater element is a fine resistive track inside the sensor that heats and cools with every journey, and after enough years and miles it can crack and go open circuit, just as an old light bulb filament does. Heater failure is often the first thing to go on an otherwise working sensor.

To test it, unplug the sensor and measure resistance across its two heater pins, on the sensor side of the connector. On many sensors the heater wires are the two of the same colour, but check the wiring diagram rather than relying on that. An open reading condemns the sensor. A reading within the specification in your service information, usually a few ohms on a conventional sensor when cold, means the heater is intact and the fault is in the supply, the wiring or the connector. Check from each heater pin to the sensor body as well: there should be no continuity.

2. A Blown Fuse or Failed Relay

The heaters are often fed from a single fuse, or a fuse and relay, that may also supply other emissions parts such as the evaporative purge valve and the camshaft solenoids. When that supply is lost, every heater on it stops working at once. The computer then sets a heater code for each sensor it can check, and on a V engine that means P0155 and P0135 together, often with the downstream codes P0141 and P0161 or the more specific heater control codes.

So multiple heater codes are not several failed sensors. Check the fuse first. With the key on and the sensor unplugged, a test light between the heater supply pin on the harness side and a good ground should light on most designs, which have battery voltage on one heater wire and the computer switching the other to ground. If there is no voltage, follow the supply back to the fuse and relay.

3. A Harness Touching the Exhaust

An oxygen sensor harness runs inches from some of the hottest metal in the car. If a clip breaks or the harness is routed badly, the insulation melts or chafes through and a heater wire shorts to ground. The short blows the shared fuse, every heater on that fuse goes dead, and the codes appear on both banks. A new fuse blows again, sometimes immediately and sometimes only once the exhaust is hot and the harness moves.

Manufacturer bulletins document exactly this, and they show why the code’s bank number can point at the wrong side. GM bulletin 24-NA-042 covers 2024 Chevrolet Silverado and GMC Sierra 2500HD and 3500HD trucks with the 6.6L gasoline engine, where P0155, the Bank 2 upstream heater code, can set along with P0135, P0030 to P0032, P0053 and others. The cause it names is a mis-routed Bank 1 Sensor 1 harness contacting the exhaust pipe, and its note warns that the fuse for the circuit may be found open. The fix is a new Bank 1 sensor and correct routing. A Bank 2 sensor would not have fixed it.

An earlier GM bulletin, PIT5336, describes the same pattern on 2013 to 2015 Buick Enclave, Chevrolet Traverse and GMC Acadia models with the 3.6L engine. It lists oxygen sensor and heater codes for sensors on both banks, including the Bank 2 Sensor 1 heater control codes P0050 to P0052, along with a purge valve code and an open Emission 1 fuse. The cause was the harness to the Bank 1 sensor chafing on an A/C hose coupling at the back of the engine. If a fuse has blown, inspect every oxygen sensor harness on both banks before fitting a new fuse or a new sensor.

4. A Connector Fault

The sensor connector sits close to the exhaust, so it suffers heat, water and road salt. Look for a plug that has partially melted or browned, terminals that are corroded or pushed back, and a locking tab that has broken so the plug works loose. Heat damage at the connector can also leave a terminal with high resistance, which can trip a heater performance check on systems that measure the current the heater draws.

If the connector is damaged, a replacement pigtail is cheap. Make sure the repaired plug is clipped back into its bracket, away from the exhaust, or the damage will happen again.

5. A Wrong or Poor-Quality Replacement Sensor

Heaters are not all alike. Sensors for different engines can have different heater resistance and power, and the engine computer expects the one it was designed for. A sensor with the wrong heater rating, a universal sensor spliced in with poor joints, or a low-quality part can set P0155 soon after it was fitted, even though the heater works. If the code returned shortly after a sensor replacement, check that the part number matches the one specified for your engine and position, and inspect any splices.

On many Toyota and Lexus models, and some other makes, the upstream sensor is an air-fuel ratio sensor rather than a conventional oxygen sensor. It has a more powerful heater, a higher price, and sometimes a different code. Older Toyota models, for example, used P1155 for this heater. The diagnosis follows the same logic, but use the correct sensor type and the manufacturer’s resistance figure.

6. A Computer Heater Driver or Software Fault

The engine computer switches the heater through an internal driver, and a heater that shorted internally can damage it by drawing too much current. Some vehicles also have software updates that change how the heater is monitored. Both are uncommon. Consider them only when the sensor’s heater measures correctly, the supply is present, and the wiring between the sensor and the computer tests good end to end, and ask a dealer whether a bulletin applies first.

How to Diagnose P0155 in the Right Order

1. Record every stored code. P0155 alone points at this sensor. P0155 with other heater codes points at the fuse, relay or a shorted harness they share.
2. Find the right sensor: Bank 2 is the side without cylinder 1, and Sensor 1 is ahead of the catalytic converter. Trace its wire to its own connector.
3. Check the fuse and relay for the heater circuit. If the fuse has blown, inspect every oxygen sensor harness on both banks for melting or chafing before replacing it.
4. Measure the heater across its two pins at the sensor side of the connector, and from each pin to the sensor body.
5. Check for power at the harness side with the key on, using a test light to ground.
6. Check that the computer switches the heater. With the engine running from cold, a test light across the two harness-side heater pins on a typical low-side switched design should light or flicker. A bidirectional scan tool can command the heater on some vehicles.
7. Inspect the connector for heat damage, corrosion and loose terminals.
8. Clear the code and do a cold start. Because the heater check runs early in a cold start, let the car stand for several hours, then drive. A fault that is still there usually comes back within the first few minutes.

What not to replace on suspicion: the catalytic converter, the downstream sensor and, when several heater codes are present, every oxygen sensor on the engine. If you want to check the oxygen signal as well as the heater, this guide to testing an oxygen sensor covers the signal wires.

P0155 Repair Cost

These are typical ranges at an independent US shop, including parts and labour.

• Upstream oxygen sensor, Bank 2: roughly $150 to $500 fitted. The part is commonly $40 to $250, with air-fuel ratio sensors at the upper end. Labour varies most: an easy-to-reach sensor takes well under an hour, while one against the bulkhead on a transverse V6 may need more time and sometimes other parts removed.
• Fuse or relay: $5 to $150, plus finding and fixing whatever blew the fuse.
• Harness repair and rerouting: about $50 to $400, depending on how much wiring melted and whether a sensor was damaged too.
• Connector pigtail: about $20 to $150 fitted.
• Engine computer: a software update usually costs a diagnostic fee; a replacement with programming can run $1,000 or more, and is rarely needed.

Use penetrating oil and an oxygen sensor socket on an old sensor, and fit the new one with anti-seize only if it does not already have some on its threads. A stripped exhaust thread turns a cheap repair into an expensive one.

Can You Keep Driving With a P0155 Code?

With P0155 alone, yes, for a while. Once the exhaust is hot, the sensor heats up without its heater and works normally, so the engine runs as it should. The cost is a slower switch to precise fuelling after each cold start, which means slightly more fuel use and higher emissions, mostly on short trips. Fix it before your next emissions inspection, which the car will fail with the light on.

If several heater codes are stored and a fuse has blown, treat it more seriously. A shorted wire against the exhaust can get worse, the same fuse may be feeding other parts of the emissions system, and a second fault can set without you noticing while the light is already on. And if the engine runs badly, the fuel trims on the Bank 2 side look wrong, or a lean code such as P0174 appears, the sensor’s signal may also be at fault, which is a separate problem.

Related Codes

• P0135: the same heater fault on the Bank 1 upstream sensor. P0135 and P0155 together point to a shared fuse, relay or a shorted harness before two failed sensors. See the P0135 guide.
• P0161: the heater fault on the Bank 2 downstream sensor, Sensor 2, after the catalytic converter. See the P0161 guide.
• P0141: the heater fault on the Bank 1 downstream sensor. See the P0141 guide.
• P0050, P0051 and P0052: heater control circuit general, low and high faults for the same Bank 2 upstream sensor, and P0059, its heater resistance. The Bank 1 equivalents include P0030 and P0031.
• P0151 to P0154: the same Bank 2 upstream sensor’s signal reading low, high, slow or inactive. These are about its oxygen reading, not its heater. P0133 is the Bank 1 equivalent of P0153, the slow-response code.

If you are not sure whether the sensor itself is failing, the symptoms of a bad oxygen sensor cover what a failing signal looks like from the driver’s seat.

Frequently Asked Questions

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