2004 Corolla with a Check Engine light on - Faulty Oxygen Sensor?

Found a few topics on this, but all were closed before anyone explained it.

I have a 2004 Corolla with a Check Engine light on.
The Toyota dealer diagnosed this as: “Oxygen Sensor Bank 1 Sensor 1 - slow to respond”.

The diagnostic code is PO133. Elsewhere, I find that this code indicates:
The oxygen sensor is faulty
The wiring to the sensor is broken / frayed
There is an exhaust leak

Haven’t noticed exhaust leakage, and I doubt the wiring is faulty.
They want $215.69 to replace it.

  1. Where is sensor 1?
  2. Do I really need a new sensor, or can I clean the old one?
  3. Where’s a reputable place to find a new sensor?
  4. Anyone who has done this before have any tips / tricks?
  5. Can I do this myself (I’m quite handy, but time challenged).


Why are you at a dealer? There’s almost never an advantage to this except for warranty or recall service, neither of which seem to apply. They don’t have any magic knowledge or special talents with your car. Variations notwithstanding, they just tend to charge more for both parts and labor and try to upsell people on things they don’t need.

So do yourself a favor and ask around for a good, locally owned, independent shop.

The first thing to do with any code like this is clear it out of memory and drive. Often it won’t come back. If it does, then worry about it. If the dealer didn’t clear it, many auto parts stores will read and clear codes for free.

If it does come back then its not easy to say what the most rational thing to do is. One route would be to go ahead & do a full diagnosis of the cause. E.g. check for exhaust leaks and check the wiring/power supply for the sensor. (The fact that you haven’t noticed an exhaust leak doesn’t mean anything. It doesn’t have to be a big one that you would hear or smell). Of course, actual diagnostics means racking up a bill for diagnostic time.

Another route would be to toss a new sensor on & see what happens. The problem there is that you could find out its not the sensor & its $$ down the drain (except some people would also say that replacing O2 sensors periodically is a good idea no matter what). This approach is called “throwing parts at the problem” and most often is not a good idea.

Anyway, sensor 1 is on the exhaust manifold or front exhaust pipe. You can buy one at any auto parts store. I have not been under the hood of an '04 Corolla but if you pop yours and look between the cooling fan way at front of the car and the engine the exhaust manifold is sticking off of the engine. See if you can follow it to the exhaust pipe. Somewhere along the way you’ll find something that looks like a metal cylinder sticking out w/ some wires on it. That threads into the exhaust pipe & you’ll see a hex-head where it meets the pipe. That’s for a wrench & there are special sockets made for O2 sensors. In principle its simple - unscrew the old one and screw in the new one. The hard part is that there is often not a lot of room to work and that those things get seized in there from heat & rust. You could end up a few hours into trying to remove it cursing, bleeding, and wishing you’d taken it to a shop.

I’m probably giving bad guidance on describing this O2 sensor for location & R&R. Here’s a video of an '01. Its probably about the same as the '04: http://www.youtube.com/watch?v=dg_S8fbtU00

This code usually means the cross-counts for the O2 sensor has slowed down or as it’s called the O2 sensor has become lazy.

To check for this a scanner that can do with real-time monitoring would be plugged into OBD port, and with engine hot that O2 sensor’s cross-counts would be monitored to see if the cross-counts flat-line. But most of the time this code indicates a defective O2 sensor.


Open your hood. Look at the exhaust side of the engine and you will see a metal shield. In the ciddle of the shield you will see a hole and in that hole is the oxygen sensor. It looks something like a spark plug but it has several wires coming out the top to a white connector.

You can buy a new sensor at a parts store for about $75 with the plug, less without but then you have to do a lot of splicing. Also get a sensor socket for your ratchet. A regular socket will not work. The new sensor will come with some anti seize on the threads, but I would get a small tube and add some more, but be careful not to get any anti-seize on the sensor head. Its easy to replace.