P0135 – Is Your Oxygen Sensor’s Heating Circuit Broken?

The abundance of electronics, sensors, and computers that have slowly assimilated into most modern cars aren’t without rhyme or reason. Through (very) thorough monitoring, analysis, and calibration of your car’s many components, these electronics can make your car more efficient, less-polluting, while also eking out as much power as possible. That is until you find problems like a P0135 error code.

At this point, you’re left wondering if all those gadgets are there simply to add further complexity for repairs. If your car is experiencing issues, plugging in an OBD2 scanner-slash-reader is a great way to quickly extrapolate what’s going on. You might encounter a P0135 error code while doing so. As well as a message, stating “Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)”.

So, what does this all mean? Well, the oxygen sensor is a device used to measure the amount of O2 in your car’s air-to-fuel ratio. Thus, enabling it to read if your engine’s running as well as it should. If you peer into an O2 sensor module, you’ll also find a heating circuit. The latter is designed to help the sensor reach its optimal operating temperatures more rapidly. If it fails, then a P0135 code appears.

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What Is An Oxygen Sensor, Anyway?

Before we get into detail with a P0135 error code, just what is an oxygen sensor, anyway? Otherwise, it’s also known as the O2 sensor or lambda sensor. The oxygen (O2) sensor is a key part of your car’s emissions control system (to find out more, check out our guide on what is the catalytic converter). You can find the O2 sensor mounted on the exhaust manifold. The latter is a series of channels and ducts where exhaust gases are vented out of the combustion chamber.

Thus, leading them into the exhaust system, and later, out of the tailpipes. The role of the O2 sensor here is to gauge the amount of unburned oxygen that exits the exhaust. In this regard, the O2 sensor can let your engine’s ECU (engine control unit) know that your combustion isn’t optimal. It’s either a bit too rich (too much fuel, not enough oxygen) or running lean (too much oxygen, not enough fuel).

Remember, proper combustion requires an ideal air-to-fuel ratio. In most cars, this ‘stoichiometric’ ratio is 14.7:1. In other words, for every 1 gram of fuel burned, it needs 14.7 grams of air to ignite it. When this ratio goes out of whack, which can be detected by the oxygen sensors, there will be some consequences. These can be summarised as:

  • Rich Mixture – In practice, this means that there could be a lot of leftover fuel during the combustion. This unburned fuel exits your exhaust, leading to increased fuel consumption and emissions output, including lackluster performance.
  • Lean Mixture – In short, it means that there may be too much air in the combustion. As a side effect of the combustion process, this air can mix in with exhaust fumes to create nitrogen oxide. It’s not only polluting but could subsequently result in poor performance, as well as engine damage.

How Does An Oxygen Sensor Work?

To prevent your car from experiencing the repercussions of an uneven air-to-fuel ratio, it has oxygen sensors. These constantly measure the amount of oxygen inside the exhaust fumes. Hence, it could be warned if your engine’s running rich or lean. Once it finds a case for either one, it’ll alert your engine’s ECU, which ultimately reduces or increases the amount of fuel to pump into the engine, accordingly.

Oxygen sensors work by providing a voltage reading to the ECU through chemical reactions. They’re able to turn themselves on and provide some voltage once they get hot. Usually, around 600°F. Their tips are also made to react with your exhaust fumes and induce a voltage through its electrodes. The voltage reading can then tell you whether the air-to-fuel ratio is optimal:


  • 0.45 Volts – Perfect stoichiometric ratio of 14.7 parts air to 1 part fuel (14.7:1)
  • 0.1 Volts – Lean (too much oxygen, not enough fuel), causing a low voltage
  • 0.9 Volts – Rich (too much fuel, not enough oxygen), causing a high voltage

In every vehicle made after emissions mandates in 1996, there should be at least two oxygen sensors. An ‘upstream’ and ‘downstream’ O2 sensor, respectively. The naming is based on the relative position of the corresponding O2 sensors to the catalytic converters. Yet, some cars have more, like sports and performance cars. With dual exhausts, for instance, you’ll need two oxygen sensors for each bank.

What Are The Up- And Downstream O2 Sensors?

So, what are the differences between the upstream and downstream O2 sensors (also, for more insight, check out our guide on what does the catalytic converter do)? For a start, there are their respective locations, and how your ECU names them, Sensor 1 or Sensor 2:

  • Upstream O2 Sensor (Sensor 1) – Mounted on the exhaust manifold, to measure the exhaust fumes coming out of the combustion engine.
  • Downstream O2 Sensor (Sensor 2) – Located behind or around the catalytic converters in the exhaust, to gauge the fumes leaving those catalysts.

The upstream O2 sensor should be fairly straightforward. As we summarised earlier, your ECU (or it’s sometimes referred to as the PCM or ‘powertrain control module‘) will read inputs from these oxygen sensors to know the air-to-fuel ratio. If it’s running lean, the ECU will start pumping more fuel into the mixture. If it’s running rich, the ECU will cut back on how much fuel to supply to the engine.

Meanwhile, there are the downstream O2 sensors. It directly measures the quality of the fumes left straight out of the catalytic converters. Primarily, it can spot if the catalytic converters are working or not, and whether they’re in good shape. At the same time, it provides the ECU with a clearer picture of the overall air-to-fuel ratio. Your engine has the tendency to flip-flop between rich and lean.

Where Does A P0135 Heating Circuit Come Into Play?

So, a bit more about that flip-flopping we spoke of earlier. This is what’s known as a ‘closed feedback control loop’. Essentially, the engine regularly alternates between running too rich and lean. This is a necessary procedure, as it’s incredibly challenging to maintain a perfect balance all the time. Thus, it goes rich for one combustion cycle, before turning lean in the next one, and so on, repetitively.

As they average out, the air-to-fuel ratio will end up being close to that ideal 14.7:1 mixture. Yet, this isn’t the same when you’re just starting your car up. With a cold engine (or if it’s the case when an O2 sensor fails), your ECU instead runs in an ‘open feedback control loop’. Here, your ECU (or PCM) can’t receive any inputs from the oxygen sensor, as they’ve yet to get hot enough to induce any voltage.

Therefore, the ECU manages the engine to run on a fixed air-to-fuel ratio, which tends to be more on the richer side of the mixture. An ‘open loop’ like this does increase fuel consumption and emissions for a moment. Nevertheless, this will balance out once the oxygen sensors can activate. Thus, prompting the ECU to moderate the amount of fuel (more or less) that’s pumped into the engine.

However, such a compromise wouldn’t be necessary for many newer cars, owing to their inclusion of a heating element in the O2 sensors. They practically heat up the oxygen sensor during a cold start. As it does so, the O2 sensors can now get to their optimal operating temperature much sooner. This can then enable you to minimize the amount of time the engine has to run in a less-efficient ‘open loop’.

What Does A P0135 Error Code Mean, Then?

When you see a P0135 ‘diagnostics trouble code’ (or DTC) appear, it’ll be accompanied by a message; “Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)”. If we use what we’ve learned thus far, it means that the heater/heating circuit in one of your oxygen (O2) sensors has failed. Specifically related to an upstream O2 sensor (Sensor 1), and one on the first cylinder bank (Bank 1 out of 2).

In OBD terms, ‘bank’ refers to the cylinder banks for those engines with a V-configuration. Such as a V6 or a V8. You may refer to your car’s owner’s or service manual to see which side of the engine is labeled as Bank 1 or Bank 2. In general, Bank 1 refers to odd numbers in the cylinder count. Bank 2 then, refers to even numbers. To specify, these would be (imagine if it’s a V8 engine):


  • Bank 1 – Cylinders 1, 3, 5, and 7
  • Bank 2 – Cylinders 2, 4, 6, and 8

The P0135 error code will appear if the ECU detects a malfunction with the O2 sensor’s heater circuit. It’s usually prompted by the ECU after having noticed erratic or odd voltage readings from the heater circuit in an O2 sensor. We’ve noted the voltage outputs of 0.1, 0.45, and 0.9 volts while reading the air-to-fuel ratio. If the O2 sensor rapidly fluctuates between them, the ECU may see it as a heating issue.

Besides that, the ECU might otherwise flash a P0135 error code if it detects incorrect current readings from the heater circuit. Generally, it’ll only appear when the heater circuit’s current stays far lower or higher than its optimal amperage. That’s especially so when those current readings remain that way for too long. There could be a variety of causes as to why it appears, though not all of it is serious.

What Causes A P0135 Error Code To Appear?

To be clear, a P0135 error code might not necessarily appear due to a fault with the O2 sensor itself. Some of the most common causes of you’re seeing a P0135 error code includes:


  • Poor connections and wiring that’s attached to the oxygen sensor.
  • Short-circuit or open-circuit between amidst the oxygen sensors’ wiring.
  • Excessive current draw within the heater circuit.
  • Problems with the ground wiring and connections.
  • Corrosion, moisture, or visible damage to the electrical connectors and pins.
  • Damaged, worn-out, or defective catalytic converters.
  • The oxygen (O2 or lambda) sensor has malfunctioned.
  • Vacuum leaks (intake or exhaust) were detected within or around the oxygen sensors.
  • ECU or PCM has failed or isn’t functioning properly, misinterpreting the oxygen sensors’ readings.
  • PCM or ECU software has glitched out or hasn’t been updated to the latest version.
  • The engine’s coolant temperature sensor isn’t working right.
  • Blown fuses and relays that tripped the electrical signals.
  • Improperly installed or incompatible aftermarket oxygen sensor.
  • The car’s battery charge (voltage) is too low for the electrics to be running right.

Most of the time, it’s probable that electrical issues are the cause for why you’re seeing a P0135 code appear in your OBD diagnostics tool. The likeliest ones include too much current draw, a short circuit, an open circuit, or a bad ground connection. Besides that, another typical culprit could be damage to the electrical connectors and wiring. For instance, frayed cables or corrosion on the pins.

How Does A P0135 Error Code Show Up As Symptoms?

When an oxygen sensor fails and throws up a P0135 error code, you’ll notice its symptoms. It isn’t as serious as some other issues, thankfully. Therefore, you can feasibly keep on driving for a little bit, if you can’t get it fixed right away. However, you’ll experience some of these symptoms, which is why a prompt and speedy trip to the workshop is nevertheless recommended:

Oxygen O2 lambda sensor problems

Typically, the most common symptoms are the illumination of the check engine light (CEL) and rough idle, as far as the O2 sensor is concerned. Granted, most of the symptoms that we’ve listed here could easily be confused for many other issues within the engine. Therefore, it’s a good idea to grab hold of an OBD2 tool and scan for any trouble codes. If P0135 appears, then you know exactly what’s wrong.

How Can You Diagnose A P0135 Error Code?

It’s worthwhile ticking away at other potential faults before you get to the O2 sensor. For example, a P0135 trouble code may have appeared due to exhaust leaks or faulty wiring. Both of which would be far cheaper and simpler to fix than having to replace the entire O2 sensor. Therefore, it’s wise to get these other components relating to the O2 sensor checked out thoroughly beforehand.

Should the P0135 error code continue to reappear despite having replaced the oxygen sensor, it may be a good idea for you to hand this over to a mechanic. At that point, it’s clear that something else is wrong, rather than a bad oxygen sensor. To get started, here are a few simple diagnoses and tests to perform, in making sure that the oxygen sensor’s heating/heater circuit is truly to blame for P0135:

Test 1: Visual Inspection Of The Oxygen Sensor

The first diagnosis that you’ll perform is a visual one. Locate the affected oxygen (O2) sensor using a repair manual. Now, you can take a closer look. Before that, it’s always a good idea to be very careful when handling electrical components around your car. If necessary, disconnect the battery terminals to cut off the electrical supply.

  1. First, check all the wiring harnesses and connectors leading to and from the oxygen sensor.
  2. Is there any fraying or cracking throughout the insulation on the wiring?
  3. See if there’s water that managed to get into the O2 sensor’s connectors and pins.
  4. Take a peek at the pins, and see if there are signs of visible damage or corrosion.
  5. Ensure that the connectors haven’t been knocked loose, and are securely in place.
  6. If you can find it, look into your car’s fuse box to see if the O2 sensor’s fuses have blown.
  7. Was there any corrosion or other damage to the engine’s ground connections?
  8. Does the O2 sensor housing contain traces of oil, dirt, or other contaminants?

Should you have spotted any signs of wear and damage to the electrical components, you can make replacements or repairs where necessary. Once all of that is done, you can use your OBD tool to clear out that P0135 code, and reset the check engine light (as you would with a BMW check engine light reset). Go for a short test drive, and make sure that the engine’s gone up to optimal temperatures. But before you do that, we can check the voltage.

Test 2: Checking The Voltage And Current Readings

Some OBD2 diagnostics tools have the ability to let you monitor a component in real-time. You could thus look at this data in person. Are you able to spot that the voltage and amperage outputs for the O2 sensor aren’t right? If that’s the case, then get yourself a multimeter, and test the sensor’s output voltage. You can do this by:

  1. Carefully disconnect the oxygen sensor connectors, before then testing the heater/heating circuit.
  2. Place the black lead of your multimeter to the ground connection, and the red lead to the connector.
  3. There should be a reading of around 12 volts. Otherwise, there’s an issue with the electrics that supply power to the sensor.

After you’ve established the power and ground circuits are all fine, you can proceed with seeing if the heater has an open circuit, short-circuit, or high resistance:

Oxygen O2 lambda sensor problems

  1. Set your multimeter to measure resistance (ohms).
  2. Attach the multimeter’s leads to the heater circuit’s pins on the O2 sensor’s connectors.
  3. Take a reading of the resistance value. Does this meet with the specifications of your car (refer to a service manual)?
  4. If the multimeter throws out an “OL” reading, it means that the heater circuit has an ‘open loop’, and has gone awry.
  5. While you’re there, you can then move on to inspecting the input voltage between the ECU and the heater circuit. Once again, refer to a service manual to note if this is within your car’s specifications.

How Can You Solve A P0135 Error Code?

If you’re lucky, you’re able to solve a P0135 trouble code for good on Test 1. Through a visual check of the electrical system, wiring, and connectors, you could sometimes fix the issue. As mentioned, this is attributed to electrical faults, such as bad wiring, loose connector, or damaged pins. These can easily be solved during the diagnosis, without needing costly replacements or repairs.

Wiring repairs, depending on the root cause, are relatively inexpensive and simple to perform. If that didn’t fix it, perhaps it was due to a blown fuse? Similarly, replacing the heater circuit’s fuse is pretty cheap. You can get an entire case of replacement fuses for as little as $10. Plus, you can easily do this DIY-style at home, saving you from pricy workshop labor charges.

If neither one works, then you’ll have to replace the affected O2 sensor. They’re typically made to last around 3 to 5 years or approximately 30,000 to 50,000 miles. Depending on the make and model of the vehicle in question, oxygen sensors aren’t inexpensive. On average, you’re looking at anywhere in the range of $100 to $500. Most of which is due to the high labor rates for a replacement job.

If you can manage this by yourself, you can find compatible O2 sensors for your car for as little as $20 online. Or, closer to $100, depending on the vehicle. Another key factor to consider is the number of O2 sensors that need replacing. With certain vehicles, you’ll have to replace both the O2 sensors at a time, rather than just the lone, damaged unit. In other words, both the up- and downstream sensors.

P0135 Error Code Facts:

  1. P0135 error code is a variation of OBD II code P0XXX and identifies a malfunction regarding the vehicle’s O2 sensor heater circuit.
  2. The code sequence identifies the problem area of a vehicle’s OBD II system, with number 1 indicating a problem with fuel and air metering.
  3. Symptoms of a P0135 error code include the illumination of the vehicle’s check engine light, rough idling and running, and lack of O2 sensor feedback until the O2 sensor gives an input.
  4. There are only two known causes that contribute to the display of a P0135 error code, which are a short or open circuit or an excessive current draw in the heater circuit detected by the ECM.
  5. A failed O2 sensor heater circuit can cause engine stalling or running inadequately, increased fuel consumption, and the accumulation of carbon.
  6. It is recommended to seek repairs from a mechanic before operating the vehicle at all, as short-distance drives may not result in any mechanical issues.
  7. Diagnosis and repairs must be done by a professional mechanic as P0135 error codes pertain to some of the most advanced and intricate parts of a vehicle.
  8. Common mistakes in diagnosing a P0135 error code include neglecting to inspect the O2 wire harness for water entry, not checking the O2 sensor for contaminants, and replacing parts before conducting thorough pinpoint tests and visual checks.
  9. Repairing a P0135 error code starts with a thorough inspection of the O2 sensor and heater circuit before replacing any sensors, fuses, or wiring.
  10. Seek an expert mechanic for repairs as a DIY approach may result in more harm than good and more money out of your pocket.

Final Thoughts On P0135 Error Codes

That then just about rounds up our thoughts on the P0135 error code. In short, we can blame this trouble code appearing on your oxygen sensor’s heater circuit. While its creation is a boon for those cars that feature one, the heating element does come with flaws. Primarily, it can malfunction, which leads to the entire O2 sensor requiring a replacement. There are ways to prevent this from occurring, at least.

Make it a habit to regularly inspect your car’s under-the-hood electrics and wiring. It’s wise to ensure that the wires don’t touch, contact, or overlap one another. Given time, this causes short circuits, and as we’ve learned, this is among the most common causes of why the heater circuit dies. In addition, a quick anti-rust coating or spray would also help prevent corrosion from eating into the wires, too.

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