P0133🌫 Emissions / oxygen sensorsDrive with caution, fix soon

P0133: O2 Sensor Circuit Slow Response, Bank 1 Sensor 1 (Meaning, Causes, Fixes)

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0133 means

Bank 1 Sensor 1 is the upstream oxygen sensor, in front of the catalytic converter, on the side of the engine that contains cylinder 1 (on a four-cylinder engine there is only bank 1). A healthy conventional oxygen sensor flips quickly back and forth between a low voltage (lean) and a high voltage (rich) as the engine computer trims the fuel mixture. P0133 sets when those switches take too long. RepairPal says the computer times the switch from below 400 millivolts to above 450 millivolts, that the exact limit varies by vehicle, and that a conventional sensor should make the switch in under 100 milliseconds; it adds that cars with a wide-band air-fuel ratio sensor in this position are monitored for the same thing using different values. OBD-Codes.com describes it more loosely: the air-fuel ratio is not being adjusted by the oxygen sensor signal as often or as expected once the engine is warm. P0139 is the slow-response code for the downstream sensor (Bank 1 Sensor 2), and P0153 is the same fault on bank 2.

Severity: can I keep driving?

Drive with caution, fix soon

Usually drivable for now, but get it checked soon. RepairPal rates P0133 Moderate: not an immediate threat to safety or the car, but it should be diagnosed within a reasonable time. OBD-Codes.com says you will likely not notice any drivability problems, while RepairPal lists rough running, stalling and worse fuel economy, so take it more seriously if the engine isn't running right. RepairPal also notes that a slow front oxygen sensor leads to higher NOx and carbon monoxide emissions, which can mean a failed emissions test.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Faulty, worn or contaminated oxygen sensor or air-fuel ratio sensor (both sources; OBD-Codes.com calls it the most likely cause if the code returns)
  2. Exhaust leak (both sources)
  3. Broken, frayed or shorted sensor wiring, or another circuit problem (both sources)
  4. Intake air leak (both sources; OBD-Codes.com lists it under things to check)
  5. Faulty sensor heater circuit (RepairPal)
  6. Low fuel pressure (RepairPal)
  7. Faulty engine coolant temperature sensor (RepairPal)
  8. Mass airflow (MAF) sensor not working properly (OBD-Codes.com lists it under things to check)
  9. Engine computer needs a software update, or a failed engine computer (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Read all stored codes and freeze-frame data. If P0133 shows up alongside lean or rich codes, other oxygen sensor codes or a heater code such as P0135, work on those together.
  2. OBD-Codes.com suggests clearing the code and seeing if it comes back. If it returns, the front bank 1 sensor is the most likely culprit, but check the items below before buying one.
  3. Inspect the Bank 1 Sensor 1 wiring and connector for melted insulation, fraying, chafing against the exhaust or corrosion, and do a wiggle test with the engine running.
  4. Look and listen for exhaust leaks ahead of the sensor, such as a cracked manifold or a leaking gasket.
  5. Check for intake air leaks after the airflow sensor: cracked hoses, a loose intake duct, or a hissing sound at idle.
  6. With a scan tool showing live data on a warmed-up engine, watch how quickly the sensor switches between low and high. A lazy sensor rises and falls sluggishly instead of flipping quickly.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The code comes back after a new sensor is fitted, which points to a leak, wiring, fuel pressure or the computer
  • You need the switching time measured properly: RepairPal describes a lab scope test where a good sensor goes from lean (under 300 mV) to rich (over 750 mV) in under 100 milliseconds
  • The car has a wide-band air-fuel ratio sensor, which RepairPal says the computer monitors using different values
  • The car failed an emissions test, or it stalls or runs badly

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0133?

Usually, for a while. RepairPal rates it Moderate, and OBD-Codes.com says you will likely not notice drivability problems. If the engine idles rough, stalls or smells rich, get it checked sooner.

Does P0133 mean I need a new oxygen sensor?

Often, but not always. OBD-Codes.com says that if the code returns after clearing, the front bank 1 sensor is more than likely the problem. Both sources also list exhaust leaks and wiring faults, and RepairPal adds intake leaks, low fuel pressure, a faulty heater circuit and a bad coolant temperature sensor, so rule those out first.

What's the difference between P0133 and P0139?

Both are slow-response oxygen sensor codes on bank 1. P0133 is the upstream sensor before the catalytic converter (Sensor 1), and P0139 is the downstream sensor after it (Sensor 2).

How much does it cost to fix P0133?

Neither source gives a repair cost for P0133. It depends on whether the fix is a new sensor, a wiring repair or fixing a leak, so get it diagnosed first.

Sources

  1. OBD-Codes.com: P0133 Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 1)
  2. RepairPal: P0133 O2 Sensor Circuit Slow Response (rated Moderate)

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.