P0133 Code: Oxygen Sensor (Symptoms & Fix)

A P0133 code is an error code typically observed in vehicles ranging from sedans and luxury cars to full 4x4s – Jeeps, Mazdas, Fords, and Jaguars are but a few examples. While the code has relatively minor implications, it cannot be dismissed or procrastinated. Doing so leads to ridiculous repair expenses and subpar (not to mention compromised) performance of your vehicle.

The P0133 code pertains to a slow-responding O2 sensor circuit. Its symptoms include an activated CEL, reduced fuel economy, and erratic engine performance (to name a few). While these indicators typically point to a defective oxygen sensor, they can also be caused by a damaged catalytic converter.

Because its symptoms are commonplace in many vehicles, a defective sensor can be easy to overlook. However, this should never be the case, as a simple O2 sensor flaw could escalate to serious engine damage or total power loss. Continue reading this guide and learn more about common causes and symptoms of a bad O2 sensor, as well as tips on how to fix and corresponding costs.

What the P0133 Code Means

Code P0133 is an error code on an issue with at least one of your oxygen sensors. It is defined as “Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 1),” read via an OBD-II diagnostic connector. The “(Bank 1, Sensor 1)” portion of its description specifies the component suspected to be at fault – which, in this case, is your heated/upstream oxygen sensor.

Depending on a car’s make and model, this fault code may be worded differently by each OEM. Here are examples of how the P0133 code displays in the OBD-II code scanner for some vehicles:

Vehicle	Code
P0133 Code Ford	O2 Circuit Slow Response (Bank 1, Sensor 1)
P0133 Code Hyundai	(O2) Circuit Slow Response (Bank 1, Sensor 1)
P0133 Code Jeep	O2 Sensor 1/1 Slow Response
P0133 Code Toyota	Oxygen Sensor Circuit Slow Response (Bank 1 Sensor 1)

Understanding How Oxygen Sensors Work

Oxygen or O2 sensors serve as gatekeepers of the quality of oxygen in an automobile’s exhaust pipe. By function, they are messengers of air-fuel ratio measurements (14.7 parts of air:1 part of fuel, on average) from different parts of an engine to the PCM/ECM/ECU. Most cars have at least two sensors – oxygen sensor #1 (which measures air going into the ‘cat’ or catalytic converter and is in the exhaust system) and oxygen sensor #2 (which measures the composition of emissions or exhaust gases and is behind the ‘cat’). They are otherwise referred to as upstream and downstream O2 sensors.

O2 sensor #1 pointedly sends air-fuel ratio data as voltage readings to the vehicle’s brain. Customarily, this voltage output changes in synch with when a driver presses on the gas pedal (among other factors). Once this data reaches the PCM or ‘brain,’ the latter assesses if the signal voltage is too high or too low (running rich or lean) and adjusts the air-fuel ratio accordingly.

Receipt of this information needs to happen in good timing. This means the sensor’s signal voltage needs to switch from 400 mV to 450 mV within approx. 100 milliseconds after the PCM has entered closed-loop operation. Otherwise, a slower-than-normal rate would get code P0133 triggered.

Common P0133 Code Symptoms

Though not all the time, your vehicle’s PCM sets off the P0133 code alongside one or more indicators. They may happen either simultaneously or one after the other. In some cases, the symptoms may occur way before the P0133 becomes triggered. That said, here are a few of the symptoms to watch out for:

1. Emissions Test Failure

Because oxygen or O2 sensors are essentially an emissions device, they take part in regulating your car’s air-fuel mixture. Both upstream and downstream sensors work together to ensure your wheeler doesn’t release harmful fumes into the atmosphere. Hence, it only makes sense that a failed emissions test is indicative of a faulty oxygen sensor and an impending P0133 code.

2. Illuminated Check Engine Light or CEL

An activated Check Engine Light is almost an expectation for error codes. But contrary to popular belief, it isn’t directly set off by a defective oxygen sensor. A bad O2 sensor upsets another system or function responsible for triggering the CEL.

Nonetheless, faulty oxygen sensors should never be taken out of the equation. They should be validated via re-scanning and accounting for accompanying symptoms. Code P0133 mustn’t be confused with other relevant error codes, such as P0131 (O2 Sensor Circuit Low Voltage) or P0136 (Heated Oxygen Sensor Circuit Malfunction).

3. Stalling or Erratic/Unpredictable Engine Performance

Rough idling, misfiring, and hesitation are rarely issues for vehicles with an up-to-spec air-fuel ratio. However, the reverse is true for cars suffering from the effects of a faulty sensor. Not only does combustion become inefficient, but performance irregularities also become more noticeable. At the very least, your car will feel sluggish, even if it hasn’t experienced hard-driving or abuse. Worst case scenario, it may just cut off and force you to pull over abruptly.

With these symptoms, you will have to check on your electrical components or your exhaust system. Inspecting your spark plug for fouling and having it replaced usually helps solve minor P0133 triggers. In the case of suspected fuel starvation, a fuel pressure test will have to be carried out first – in addition to checking on the condition of fuel system components.

Whatever you do, what’s important is not to delay acting on the problem. Doing so prevents bad O2 sensors from worsening into speed stasis, sputtering, restricted acceleration, or loss of power.

4. Bad Fuel Economy/Gas Mileage

While typically indicating a compromised carburetor or fuel system components, poor fuel economy is also a sign of bad oxygen sensors (or worse, a failing PCM). Bad fuel economy is mostly due to an engine running too lean or rich and is characteristic of O2 sensors giving off incorrect voltage output. This telltale sign warrants urgency, especially if the fuel economy reduces drastically or abnormally.

5. Unusual Engine Noises

When O2 sensors fail as an emissions device, it unintentionally aids in carbon buildup in the exhaust system and the combustion chambers. Either situation impedes the oxygen sensors from doing its job and sending the right signal voltage to the PCM on time. Eventually, knocking/pinging noises occur due to faulty sensors leaning towards a lean fuel mixture.

6. Black Exhaust Fumes

As established earlier, combustion efficiency is one facet of your vehicle systems adversely affected by a defective O2 sensor. When inefficient combustion occurs, it leads to many other problems such as backfiring, unburned fuel, poor idling, starting difficulties, and soot-like/black smoke residue. Although non-harmful emission is, for the most part, a job of O2 sensor #2, the upstream O2 sensor controls the quality of air processed by the engine and combustion chamber.

7. Sulfuric/Rotten-Egg Smell From the Tailpipe

This putrid smell from the exhaust usually occurs alongside black exhaust fumes and is likewise a sign of bad oxygen sensors. Specifically, a sulfuric or rotten egg smell means excessive fuel inside a vehicle’s engine – which traces back to incorrect air-fuel mixture and equally incorrect voltage measurements sent to the PCM. This indicator can be pretty tricky to diagnose since it can also signify problems with fuel system components. So if you get this symptom, be sure to look into both your car’s fuel system and oxygen sensors.

8. Engine Overheating

Despite being less common, it is one of the symptoms in this list with more serious repercussions. Engine overheating rarely happens as an aftermath of faulty O2 sensors. But when it does, it only means that the stock O2 sensors have been pushed to (or well beyond) their limits. Early signs of the problem have been disregarded or overlooked. These situations ultimately result in O2 sensors that are extremely worn and screaming for a replacement.

9. Premature or Sudden Catalytic Converter Failure

‘Cat’ or catalytic converters are part of a car’s emissions system. They share a rather similar function with O2 sensors while also relying on the latter, in the sense that they control air in the exhaust and reduce toxic fumes. A premature ‘cat’ failure signifies faulty O2 sensors that have long been neglected. That said, they’re the one thing you wouldn’t want to get dragged into your vehicle’s O2 sensor problem, as they’re quite expensive to get replaced or even fixed.

Adherence to periodic maintenance tremendously helps keep ‘cat’ failures at bay. However, you’ll know the inevitable once you encounter any one of these other symptoms:

• Physical damage or corrosion on the catalytic converter
• Compression misfiring due to leaky valves or head gasket
• Internal coolant leaks from cracks in the head gasket
• Ignition misfiring due to a shorted plug wire or fouled spark plug
• Oil burning due to worn valve guides, rings/seals, or cylinders
• Fuel contamination

What Causes the P0133 Code?

There are several ways for an oxygen sensor to go bad, and it’s not just because you have an old vehicle. Inefficient fuel combustion and poor engine performance are but some of the adverse effects of having a faulty O2 sensor – which is why it’s never a good thing. To prevent your upstream O2 sensor from failing prematurely, make sure to look into the following at the first encounter of a P0133 code:

• Defective oxygen sensors
• Physical damage to the actual O2 sensor or the area where it is located (along the exhaust system, exhaust manifold, or catalytic converter)
• Saturated/frayed wiring to the O2 sensor; shorted/open circuits and blown fuses
• Carbon buildup in the air intake and fuel systems resulting from dirt, grime, or debris
• Engine vacuum or exhaust leaks that contaminate the sensor’s reference air or allow unmetered air into the engine
• Dirty MAF (Mass Air Flow) sensor
• Faulty ECT (Engine Coolant Temperature) sensor
• Incorrect/low fuel pressure caused by incorrect tank venting, weak pump, dirty fuel filter, etc.
• Low battery voltage results in problems with the heater control circuit and low voltage readings from the O2 sensor (although rare, fault codes P0030 and P0032 may also show up)
• Failing or defective PCM (Powertrain Control Module) or outdated software
• Dismissing early signs of faulty O2 sensors
• Use of low-quality/discount fuel (or one with an incorrect or lower Octane rating) resulting in fuel contaminants, excessive oil burning, and clogging
• Extended use of fuel additives
• Non-adherence to OEM-recommended oxygen sensor replacement schedule
• Poor vehicle maintenance or a disregard for periodic change of air filters, fuel filters, spark plugs, etc. translating into caked/gunked O2 sensors
• Not having a full vehicle inspection at least once a year

When Should Oxygen Sensors Be Replaced?

O2 sensors naturally become caked with combustion by-products as a vehicle ages. Hence, automobiles less than 15 years old should have their oxygen sensors professionally replaced every 60,000 to 100,000 miles or 7–10 years. Conversely, older cars require more frequent intervals – every 45,000 to 65,000 miles (sometimes even 30,000 to 50,000 miles) – to keep their mills running smooth and their exhaust fumes within acceptable levels.

Delaying on addressing a P0133 code is never an option – nor is continuing to drive with a bad O2 sensor. Not only is doing so counter-intuitive to your engine’s health and performance, but it also puts you and your passengers at risk while on the road.

Diagnosis and P0133 Code Fix (7 Tips)

1. Check TSBs

Check related TSBs (Technical Service Bulletins) for existing issues specific to your car’s O2 sensor.

2. Fix Other Codes First

Plug an OBD-II Scanner into the OBD port of your vehicle and check for other error codes apart from P0133. If there are, tend to them first. Make sure to read the freeze-frame data and document your findings when doing this step. Clear the other codes in their stored order and see if it lifts the P0133 code. And preferably, use a higher-spec OBD-II tool that can effectively update live data streams.

3. Check Wiring

Visually inspect the O2 sensor’s wiring and connector for fraying/saturation (from contaminants) or disconnection, test for open circuits and shorts, and immediately replace if any are found (consult your owner’s manual for color-coding and routing specifications). You may also want to check for damage to the sensor and its surrounding area while you’re at it. If physical damage is spotted, you will need to replace the O2 sensor itself.

4. Test the O2 Sensor

Perform ground, continuity, reference voltage, and resistance checks on the suspected O2 sensor if its wiring and connectors show no visible damage. Don’t forget to disconnect the sensor from the PCM before conducting these checks (especially resistance and continuity) to ensure no damage to the latter. Compare your readings and those in the manual, and see if actual readings are up to spec or the sensor needs repairs. Where needed, do ‘back probing’ to ensure that the PCM gets signals from the O2 sensor. (Note: If your PCM has historically dismissed minor engine misfires, it may need a tune-up too.)

5. Check Heat Stress

Look for heat stress, soot saturation, or oil fouling at the base of the oxygen sensor. Check the O2 and MAF sensors for dirt and corrosion. Clean them with a MAF sensor cleaner while ensuring all wires are still firmly connected. Reinstall the MAF and O2 sensors, clear the P0133, and re-scan to see if the fault code returns.

6. Look For Leaks

Check for leaks in the air intake, exhaust, and vacuum hoses. Pay mind to the ends of hoses where fraying or looseness is likely to occur. If there is no fraying, ensure all hoses are firmly connected. Using an EVAP smoke machine around the vacuum hoses and in the fuel tank is ideal in spotting leaks. A less orthodox approach is to ‘wand’ an unlit propane torch over the air intake and vacuum lines while idle – fluctuations in engine RPM would direct you to the leak source.

7. Measure the Voltage Output

Measure the oxygen sensor’s voltage output with an OBD-II scanner or a digital multimeter. Connect the latter’s positive lead to the sensor’s connector, and its black lead to a ground point or the negative battery terminal. Have someone rev the engine while you add fuel – the oxygen sensor’s voltage should go up as you do this. If there is no voltage spike, chances are something is wrong with either the sensor or its wiring.

Fouling, in particular, needs to be examined more closely. Oil fouling on O2 sensors, for instance, leaves a sticky, tar-like residue and has an almost similar dark color as a rich fuel mixture or lead poisoning. Silicone poisoning and antifreeze contamination have a much lighter hue but are equally problematic (if not worse). Any one of these can inhibit an O2 sensor’s ‘sensing ability’ and response time.

If you discover silicone-based compounds fouling your oxygen sensor, make sure to immediately stop using all aftermarket oil and fuel additives and never use variants with silicone-derived compounds moving forward. Continued use of such substances will not only result in a persistent P0133 code but also eventually destroy your vehicle’s catalytic converter. Similarly, the root cause of other kinds of fouling needs to be identified and addressed. Otherwise, replacing your O2 sensor would prove futile.

Cost of Clearing/Fixing Code P0133

Some licensed mechanics can run an engine code diagnostic on your car for $95 to $110. A few charge more, while others do this for free. Rates usually depend on how many error codes go with the P0133, as well as your vehicle’s make and model. As for labor costs, the charge is usually $35 – $200 hourly or a minimum of $100. These rates may change, depending on difficulty accessing the O2 sensor and the number warranting replacement (but barring complications).

By itself, an O2 sensor replacement falls on the cheaper end of the repair expense spectrum. Collectively, however, fixing a P0133 code (including diagnostics, relevant components, and labor) may range from $500 to $2,500. This value doesn’t yet include damage to the catalytic converter – a known outcome of O2 sensor problems too far gone. Nor does it account for OEM vs. aftermarket replacements.

The below table provides estimated repair costs associated with clearing the P0133 code:

Emission Control System Repairs

Component	Estimated Cost
Gas Cap O-ring	$10 – $50
Gas/Fuel Filler Cap	$20 – $80
Exhaust (if welded)	$100 – $200
MAP (Manifold Absolute Pressure) Sensor	$150 – $320
MAF (Mass Air Flow) Sensor	$80 – $480
Oxygen Sensor	$200 – $500
Vacuum Feed Line	$100 – $1,000
Fuel Tank	$400 – $1,100
Fuel Pump	$345 – $1,320
PCM or ECM	$800 – $2,000
Catalytic Converter	$200 – $3,750

How to Replace an O2 Sensor

Replacing your upstream oxygen sensor is inevitable, especially if it’s worn out to the core and can no longer be repaired. However, it is an intermediate DIY task that not everyone can undertake. Follow the steps detailed in this section if you’re confident in doing it on your own. Otherwise, seek assistance from a professional mechanic to do the O2 sensor removal and installation for you.

10 Steps:

1. Use an OBD-II code scanner to detect the faulty sensor and retrieve other potential error codes.
2. If the car has been used, let it cool down first for at least 30 minutes before removing the O2 sensor.
3. Locate O2 sensor #1 at fault (usually found on the exhaust pipe, next to the motor under the hood).
4. Disconnect the electrical connection by pulling the plastic plug from the end of the sensor back by hand.
5. Loosen the faulty O2 sensor by spraying some penetrant onto it and waiting 10 minutes.
6. Unscrew the defective O2 sensor using a ratchet wrench and oxygen sensor socket.
7. Before installing the new O2 sensor, apply a small amount of anti-seize onto its threads.
8. Secure the replacement oxygen sensor on the exhaust line by turning it clockwise.
9. Plug the O2 sensor’s electrical connector back into the outlet port by pushing it into place.
10. Start your vehicle, go for a spin, and see if the new O2 sensor clears the P0133 code.

Although the above steps seem easy, removing the old sensor can prove challenging. The reason behind this is that the defective O2 sensor has been previously subjected to countless heat/cool cycles and is often nearly seized into its threads. Thankfully, a good amount of penetrating oil can help loosen it, making it less of a struggle to wrench the old sensor out. Using extensions, adapters, or a universal joint in some situations can help get hard-to-remove O2 sensors out.

Conclusion – P0133 Code Symptoms & Fix

 An oxygen sensor’s response times may vary between sub-zero weather and WOT (or wide-open-throttle) conditions. However, these timing differences are pretty insignificant and shouldn’t result in a noticeable lag or, more so, a triggered P0133 code. That said, it is always best to be prompt when zeroing in on the problem – even on the earliest signs of a bad O2 sensor. Doing so can save you and the condition of your vehicle from snowballing into worse situations.