Code P1141 - Downstream 02 Sensor - Please help!

Hi there,

So I have a 2000 Mazda Protege 1.6L engine if it doesn’t already show that on my profile.

The CEL code is P1141: Downstream

This is a link I came across from doing a google search: suggesting to change the downstream 02 sensor, however I already installed a new one in August!

My friend did some voltage testing:

  1. 5V coming out of the heating circuit
  2. No voltage coming out the PCM! --------->> What does this mean?

So what else should I check , before I go and buy yet again another 02 sensor???

Previously I had a po420 cat code, but that didn’t come up this time.


you might want to find wiring info on your car and trace wires/connections between the sensor and ECM, check connectors for oxidation, etc…

I could easily get dealer repair manuals for my Nissan vehicles, likely you will find Mazda ones online too: they usually contain step-by-step troubleshooting procedure for your particular OBD code

O2 sensors have to be hot to work, so many of them have an electric heating coil wrapped around them which heats them up so they start working faster after starting the engine, which reduces the car’s emissions. The O2 sensor still works without the heater working b/c it gets hot just by being in the exhaust stream, but it takes a little longer to start working is all. So your Mazda probably runs like it always did right? If so, this isn’t an emergency.

B/c it is emissions related, even tho it only affects emissions level for the first few minutes after starting the engine, the EPA probably requires the heating coil work. The engine computer outputs the voltage to power the coil, and monitors the voltage to make sure it is working. Somethings wrong with all that. From what I understand you’ve verified the heating coil voltage at o2 sensor, but it isn’t getting back to the engine computer. The things that could cause that,

  • corroded or broken connecter at the O2 sensor
  • corroded or broken connector at the engine computer
  • broken or shorted wire between the O2 sensor and the engine computer

So it is like if your friend starts from their home but never arrives at yours. What would you do? Go to their home and trace along the path on your map they’d take, step by step, to find out where they went missing. That’s what you need to do for your O2 sensor problem too. Map is the key word, as this could be near impossible to do without one. Secure a wiring schematic for your car, and have a go at it. Or if you aren’t familiar with how to do that kind of thing, ask your local auto electric shop to have a go at it, using your wiring schematic. Best of luck.


Are you sure you connected your multimeter to ground and sensing wire? The voltage should measure in millivolt, between 0.10 to 1.0.

1 Like

Hi George,

What are the chances that the root cause is faulty cat?

God, I hate how parts are only designed to last 10 years.

Thank you sir,


It’s definitely possible on a 18 year old car, especially if you’re clocking over 200K miles. But if you had a faulty cat you’d probably be getting a consistent p0420, which you said in the OP had disappeared. The p1141 is for the heater circuit for the O2 sensor right? A faulty cat wouldn’t normally cause that problem. A p1141 code could cause a false diagnostic code for the cat I suppose. In other words the faulty heater circuit could affect the O2 sensor’s reading, and that could make the computer think the cat is bad when it isn’t. Suggest to solve the p1141 first, before considering the cat.

Hi there George,

So I am looking at the connector there are three wires: blue, white, and black…which ones you want me to test?


Start by measuring the resistance of the o2 sensor’s heater. What I’m seeing, you disconnect the electrical connector, the downstream connector, you look at it its face, it has a little protrusion at the top. All this is done on the o2 sensor side, not the harness side, with it disconnected. So you are only testing the sensor resistance, nothing else. There’s 4 terminals that go a,b,c,d starting at the top/left and going counter-clockwise. Measure the resistance between the C and D terminals (the two right most as you look at it). Does it measure appx 16 ohms?

From what I’m seeing here the two wires for the resistance heater are colored B/L and G/R on the harness side. It doesn’t say what color they are on the sensor side.

they are blue white and black on the sensor side

I’m confused why there are only 3 wires on the sensor side, as the wiring diagram I was looking at I believe shows 4. What color are the wires on the harness side of that connector. And how do they match up to the blue, white, and black wires on the sensor? In any event, only two of the 3 wires should show a low ohm resistance between them on the sensor side. What do your resistance measurements show?

The sensor is attached to the hottest part of the exhaust in a confined space under the floor with heat shields around it that get hot enough to melt the insulation off wires. Very often that is where the problem is found. Of course if a short occurs there it can cause something else to fail elsewhere but yes, start at the sensor and work your way forward and repair as needed. There’s not much else to do.

We put the car on the hoist, and saw the wires coming out the sensor…they look good.

But I didn’t trace it back to the top of the engine under the hood

Was the wire lead on the old sensor possibly damaged?

what about doing this? I only plan on keeping the car for a year

It might work provided the sensor is the problem. Seems unlikely cleaning the surface of the sensor would fix a sensor resistance heater problem though, but if you got the sensor out already, worth a try I guess.

since you are keeping car only ofr a year, why don’t you use something like this?

the connector is mounted in a tight location that I couldn’t pry off…

but on what seemed like the right side of the four wire connector…I got 13.2 Ohms.


I have an upstream connector that measures 16 Ohms… but will have to put in some bullet connectors to lengthen the wire…

the connector is mounted in a tight location that I couldn’t pry off…

but on what seemed like the right side of the four wire connector…I got 13.2 Ohms.


I plan on selling the car when it hits 280 000, and I have 242 900 now.

Its all about determining how long the car will last for…

lots of rust, even rust on the engine mounts… hard to say.

Like an idiot I dropped $200 to change the timing belt (which needed changing), but I from this point forward I am not putting in more than $100 for any repair.

the sensor I pointed to you is $14 delivered

all the fiddling around takes time and $$

if you want to fix it and determined sensor is at fault, $14 and guaranteed fix is better than trying to revive an old sensor which should have been replaced long ago

your call, off course…

1 Like