Seeing a P0161 trouble code in that OBD scanner of yours? The good news is that it’s not too serious and unlikely to affect your car’s operation. The bad news is that you’ll still need to repair the problem to prevent engine damage in the long run.
Additionally, this is usually an electrical issue that can be difficult to diagnose and repair. The other good news is that we will help you understand everything you need to know about this trouble code. Use our table of contents to find the information you need:
The P0161 is a generic trouble code that translates to “O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2).” Since the code has a ‘0’ as the second digit, this means it’s a generic or universal code. And it indicates the same issue on any car with an OBD-2 system regardless of make and model.
The O2 sensor is important for the car, and it helps the car to run smoothly. What a lot of people don’t know is the sensor also has a heater. But we’ll get to that in a minute, here’s how the O2 sensor itself works if you didn’t already know:
How The O2 Sensors Work
Cars have at least two O2 or oxygen sensors. One sitting before the catalytic converter, also known as the upstream sensor. And one after the catalytic converter, known as the downstream O2 sensor. In some cars, there are two of each.
Their task is exactly as the name suggests: to measure oxygen levels inside the exhaust gases. The upstream sensor measures the gases after it escapes the engine. Meanwhile, the downstream sensor measures them after the exhaust gases after filtered by the catalytic converter.
The O2 sensor’s function is twofold. First, it lets the Engine Control Unit (ECU) knows whether the engine is running on a lean (too much air) or rich (too much fuel) mixture. The engine needs to run around on a 14.7:1 air-to-fuel ratio. And too much of either one can cause operational issues and even engine damage.
If the O2 sensor is getting an oxygen level reading outside of the accepted value, then it will let the ECU know to adjust engine settings accordingly. And if the ECU can’t achieve the correct ratio, then it will trigger a check engine light, signaling something is wrong.
Its second function is to measure the efficiency of the catalytic converter. This is why there are two of them and one of them is after the catalytic converter. If the reading is outside the accepted value, then it will throw a trouble code indicating there’s a catalytic converter issue.
So, now you know what the O2 sensor is for. But what’s the O2 sensor heater circuit? Let’s break down the trouble code further:
O2 Sensor Heater Circuit
Something that not a lot of people talk about is that the O2 sensor needs to get up to temperature to work properly. If it’s not at the correct temperature—around 600 °F—it won’t work and will give incorrect readings. Hence the need for a heater to get the sensor up to temperature as soon as the car starts.
After the car warms up, the catalytic converter’s temperature will also rise. The heat from the adjacent catalytic converter will keep the O2 sensor at operating temperature, and the heater can switch off. This code means that the issue isn’t caused by the O2 sensor itself, but rather by its heating elements.
If the ECU detects an open, short, or resistance from the sensor outside of the accepted value, the heater will not work and throw trouble codes.
Note that this heater is not a separate device. Rather, the heating elements are built into the O2 sensor itself. There are a couple of types, but the gist of it is that the heater is built into the O2 sensor.
Bank 2 Sensor 2 Upstream or Downstream
Let’s break the code down further. If your car has a V-layout engine, such as a V6, V8, or a V12. These V engines have two cylinder banks. Bank 1 is always where cylinder number 1 is located, and bank 2 is opposite bank 1.
The exact location varies depending on the car’s make and model. For example, bank 1 in Ford cars is usually on the passenger side. Meanwhile, Chevy cars usually have bank 1 on the driver’s side. You can easily find this information for your specific car online.
Cars with a V engine that sits longitudinally (running lengthwise) usually has two catalytic converters, one for each bank. Sensor 2 refers to the downstream sensor. So, ‘bank 2 sensor 2’ in the code indicates the issue is with the downstream sensor of bank 2.
For example, if bank 2 is on the passenger side, then the O2 sensor heater that’s experiencing the problem is the downstream sensor on the passenger side of the car. You’ll find the sensor somewhere behind the catalytic converter on the passenger side. Now you know how to locate the sensor.
If you have a transverse V-engine or an inline engine, you may also see this code. In this case, the ‘bank 2’ refers to the second catalytic converter. In some cars, such as early 2000s Toyota and Lexus, the four-cylinder engines have two exhaust manifolds and catalytic converters; one each for every two cylinders. Check your service manual to locate it.
Most other trouble codes come with a laundry list of troubling symptoms that affects driving. But as mentioned, the P0161 code isn’t too serious. So, there really aren’t any symptoms or signs that you have a bad O2 sensor heater. This is because while the heater is faulty, the O2 sensor itself is fine—and that’s what’s important for the engine.
The engine might run a bit rough on cold starts since the O2 sensor doesn’t work when it’s cold. When the O2 sensor can’t give accurate feedback to the ECU, it will have difficulty finding the appropriate settings for the engine to run.
However, as soon as the engine and catalytic converter warm up, the O2 sensor will work fine and the engine will run as usual. And yes, this means you can get away with driving with the P0161 code. But we recommend fixing it within a month to avoid further issues and damage.
Other symptoms you might experience are an increase in fuel consumption and increased emissions. But again, this is unlikely to happen once the engine warms up. And you’re unlikely to notice it as well.
P0161 Causes & Diagnosis
There are five possible causes here:
- O2 sensor heater power circuit is open or damaged.
- O2 sensor heater ground circuit is open or damaged.
- Blown fuse.
- The O2 sensor itself is faulty.
- Faulty ECU throwing false trouble codes (least likely).
That last one is less likely, as ECUs typically don’t fail unless it’s been damaged, but it can happen. Either because the car’s been through an accident or the components of the ECU are compromised by moisture or dirt. We wrote a complete guide about ECU/PCM reprogramming and you can learn more about ECU damage here.
The first two causes are usually due to damage to the wiring. Sometimes road debris can damage them, or the connector may have also gone bad. And finally, it’s possible the O2 sensor itself is faulty. But in this case, you’ll likely see other trouble codes as well. This brings us to how to troubleshoot the problem…
P0161: How To Diagnose
Before you spend money on replacing parts, it’s always a good idea to diagnose the problem further to find the exact cause. Trouble codes don’t tell you exactly what’s wrong with the car. But it’s there to notify you of a potential problem and helps to diagnose it.
Note that this is an electrical issue, and it’s usually very difficult to diagnose—let alone fix—unless you have good electrical knowledge and skills.
So, if you’re not entirely confident, we recommend having a trusted repair shop do the diagnosis for you. They should be able to diagnose it quickly and avoid misdiagnosing the problem. Most diagnostic jobs cost around $120, provided no major disassembly job is required, which is the case for this particular issue.
If you don’t have a trusted auto repair shop, check RepairPal’s certified shop program. RepairPal.com is an auto repair information provider, and they generally have a good track record. Their certified shop program also provides a 12-month/12,000-mile warranty on repairs.
Now, if you want to diagnose the problem yourself, here’s what you need to do:
Step 1 – Scan For Other Trouble Codes
The first step is to always scan for other trouble codes in the car’s On-Board Diagnostic (OBD) system. Other codes indicate different issues with the car, and they may be the ones that caused the P0161 code in the first place.
For example, the P0156 code indicates a sensor malfunction with the O2 sensor’s circuit. Specifically, the downstream sensor of bank 2 (bank 2 sensor 2). This likely means the sensor itself is faulty, not just with the heater.
Another thing we recommend is clearing the OBD of the trouble codes to get rid of the check engine light. As mentioned, this could be an ECU error. If the light doesn’t return, then it was likely an ECU error. But if it returns, let’s move to our next step:
Step 2 – Check The Fuses
There are several fuses for the O2 sensor heater. Checking the fuse itself is quite easy, but you’ll need to locate them first. Most cars have their fuse box in the engine bay, which is a black plastic box with several clips. Simply undo the clips, take the cover off, and there should be a diagram of the fuses inside the cover.
Afterward, you will likely need a service manual to find the fuse for the O2 sensor heater. There should be plenty of information online on which one it is for your specific car make and model.
You can take it out and visually inspect it for damage to the pins or inside the fuse. You can also shake it and if it makes a rattling noise then you have a blown fuse and you’ll need to replace it.
A more accurate way though is to use an automotive test light. Simply connect it to a ground wire or the negative terminal of the car’s battery, and touch the fuse’s pins. These pins are exposed so you can easily touch them. If the test light doesn’t light up, then the fuse is blown. Replacing your fuse should fix the P0161 code.
Step 3 – Locate And Inspect The Wiring
As mentioned, the P0161 is likely caused by a wiring issue. Sometimes there’s visible damage to the wiring due to road debris, causing the O2 heater to fail and triggering the trouble code.
So, get underneath your car, locate the sensor and wiring, and look for visible damage. As mentioned, the sensor sits behind the catalytic converter that’s connected to bank 2. The sensor is a plug (kind of like a spark plug) that connects to the exhaust pipe with an electrical connector attached to it.
Inspect the wiring for signs of damage. Some cars may have a braided sleeve for the wiring. If there’s no damage to the sleeve, try opening it for signs of damage to the wires inside of it. If there are no signs of damage, time to check the oxygen sensor itself:
Step 4 – Sensor And Wiring Test
Testing wires and electrical connections are where it gets tricky. This requires time, patience, know-how, a multimeter, and a power probe. The problem can either be a faulty connector (either on the oxygen sensor or the car’s wiring), or the wires themself.
The video above is a complete guide on how to test the wiring for the O2 sensor heater. We recommend watching it if you want to diagnose the problem yourself.
You’ll need to test out the ground wires for the heater. Additionally, you’ll need to test the electrical connectors on both the wiring and the O2 sensor with a multimeter. Note that the Ohm resistance may differ depending on the car’s make and model, so check with your service manual to find the correct resistance rating for your car.
Also note that a bad ground strap can also cause electrical issues in your car, including the O2 sensor heater. So you might want to check that out as well. Learn more in our guide to engine ground straps.
P0161 Repairs & Costs
Now that you’ve found the problem, it’s time to repair it before it leads to expensive engine damage. The cost will vary depending on the repairs that you need to do:
- Fuse replacement is usually no more than $20 each, and you can easily do it by yourself. Specialty fuses may cost up to $100, but this isn’t the case for the O2 sensor heater. Just make sure you use the correct amperage.
- Wiring repairs will be necessary if you have damage. This costs $200 – $300 on average, although it may be more or less depending on the car’s make, model, and extent of the damage.
- O2 sensor replacement may be necessary. This costs up to $200 including labor.
Note that you need to keep an eye on the fuse after replacing it. If it blows again not long after replacing it, you may have used the wrong amperage. It can also mean a short in the wiring system, which can be caused by frayed wiring insulation and a faulty electrical accessory amongst other things.
We recommend leaving wiring repairs to professionals. Unless you have knowledge and experience in electrical engineering, wiring repairs are typically very difficult to do and easy to get wrong.
Meanwhile, the O2 sensor replacement is a lot simpler to do. The sensors themselves usually cost no more than $100, and you can save money by replacing them yourself:
O2 Sensor Replacement DIY
There are two scenarios where you might need a new O2 sensor: first, the sensor’s electrical connector is bad. Or secondly, the heating elements inside the sensor have gone bad. Whatever the case, if you need a new sensor, here’s how to replace it:
- Get hold of an O2 sensor socket. The size varies on each car, so make sure you get the right one. Some big auto repair shops may rent this socket.
- Slide the socket onto the sensor, then attach it to a ratchet.
- Unscrew the sensor and pull it out. Be careful if you’re working with a hot engine.
- Prepare the new O2 sensor. Rub anti-seize on the threads of the sensor, and be careful not to get any on the sensor itself. Just put a little on the thread and rub it around the thread with your finger.
- Insert the sensor into its plug, and hand-tighten the sensor.
- Use the O2 sensor socket to fully tighten the sensor. There’s no torque specification, just make sure that the new sensor sits snuggly.
- Clear the trouble codes from your car’s OBD with an OBD scan tool to reset the check engine light.
- Turn on the engine, and take a drive for 10 – 15 minutes. If the check engine light doesn’t return, you’re good to go.
The video above from ChrisFix shows how to replace an oxygen sensor. While the sensor that was replaced in the video is the upstream sensor, and the location is in the engine bay for that particular Toyota, the steps for other cars are the same.
The main difference is the location of the sensor. In the case of the downstream sensor, you’ll need to get underneath your car and locate it behind the catalytic converter. The other steps are the same.
Any more questions about the P0161 code? Here are some answers that you might find helpful:
How Do I Fix Code P0161
The fix for code P0161 in your car depends on what triggered the code in the first place. This can either be damaged wiring, bad electrical connectors, faulty downstream oxygen (O2) sensor, or something as simple as a blown fuse. A blown fuse is the simplest to fix and usually costs no more than $20. A new oxygen sensor typically costs $200 including labor. And wiring repairs cost between $200 and $300 depending on the extent of the damage.
What Is Code P0161
Code P0161 is a generic/universal Diagnostic Trouble Code (DTC) that signifies an issue with the heater of the downstream oxygen sensor in bank 2. The downstream sensor is the one that sits after the catalytic converter. And bank 2 may refer to either the engine’s second bank or second catalytic converter, depending on the make and model. The sensor itself has a heating element inside of it, which warms up the sensor to around 600 °F so that it can work properly. If the heater doesn’t work, usually due to wiring issues, then the ECU will trigger the P0161 code to alert the driver.
What Does Code P0161 Mean
Code P0161 translates to O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2). The O2 sensor measures oxygen levels in the exhaust gases, and it uses a heater so that it can warm up quickly to work properly. ‘Bank 2’ can refer to either the engine’s second bank or the second catalytic converter bank, depending on the engine type. While ‘Sensor 2’ refers to the downstream sensor which sits after the catalytic converter. This code means the heater in this particular O2 sensor isn’t working.
What’s The O2 Sensor Heater Resistance Specification
You can test the O2 sensor heater with a multimeter and test the resistance to see if it’s working properly. A good sensor should read 10 – 25 ohms, but this can vary and you should check with your car’s manual to find the exact specs.
P0161: Wrap Up
To summarize, the P0161 code signifies an issue with the downstream O2 sensor’s heater circuit. Specifically, with the downstream sensor in bank 2.
The O2 sensors need to warm up to work properly, hence why it has a heating element inside. When the car starts, the heating element will warm up the sensor. And once the catalytic converter gets to operating temperature, it turns off and the O2 sensor stays warm from the catalytic converter’s heat.
If the heater fails, then the ECU will get an erratic reading and this triggers the P0161 code. The repairs will depend on the cause, and it includes wiring damage, bad connectors, blown fuse, and faulty heating elements inside the sensor.
The cost for these repairs is anywhere between $20 and $300. Wiring repairs are the most expensive, sometimes costing $300 or more depending on the extent of the damage. Leave repairs to a trusted repair shop if you’re not sure about your DIY skills. Hopefully, this has been helpful and good luck!
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