Tell me all about Oxygen Sensors

[Station]

O2 sensors, Oxygen Sensors Lambda Sensors and all that.

 

I believe the reason why my Mazda is running like a big old bag of shite is because of the o2 sensor.

The O2 sensor only operates 'closed loop' at rev range on idle/below 3000 rpm etc and that is where my car is lumpier than a freshly repaired road by a Wirral Borough Council contractor. As soon as it gets out of that rev range it runs A1. It also failed MOT in August on emissions (to be honest, the tester didn't even test it as it filled the garage with petrol fumes). I've had the fear since and didn't want to go for another test until this problem is sorted, I heard overfuelling can cause further wear to the rotor tips.

Everything else has been checked as it fine.

 

I'm looking at O2 sensors, and they all look similar (universal) but as soon as you attach RX7 to the search terms it adds a nought onto the end.

Some ebay sellers sell specific sensors for the car, but it just looks like the usual farming auction affair.

 

Are these sensors mostly the same? They seem to run between 0.5V and 1V.

[SiC]

If a narrow band sensor (almost certain yours would be, given the age) you can go generic and they're all pretty much the same (bar dimensions). Generic will require chopping the old plug and a bit of soldering to attach the wires on. Model specific had the correct connector on.

 

Go genuine Bosch/etc. Cheap random brand aftermarket ones are invariably shit.

 

A working lambda when hot will switch from around 0.7v to 1.3v when closed loop. You can either measure that with a DMM or diagnostics tool if you have one that'll connect (your car will probably be pre-OBD2 though).

[Station]

That's good to hear, thanks.

I'm 99% certain mine is goosed, it has all the symptoms. I may try cleaning it as the car hasn't had a lot of use, but was in good running order before I put it away.

[twosmoke300]

Don't most sensors switch from 0.1 to 0.9v ?

[wuvvum]

Newfangled tosh.  Buy a Rover P6 and never have to worry about such rubbish again.

[/Junkman]

[Dave_Q]

Most switch between 0.1v and 0.9v.

 

Basically they all work on the same principal, most are 4 wires where 2 are a power supply for a built in heater to get it working ASAP when the engine starts.

 

Some are 3 wires, these either common the earths or earth the heater through the exhaust system.

 

I needed one for my Subaru for the last MOT, proper ones are BARE LOOT FAM so I fitted a 3 wire Bosch universal like this - it fixed my emissions and the car hasn't blown up yet.

[SiC]

  On 13/02/2017 at 21:08, twosmoke300 said:

Don't most sensors switch from 0.1 to 0.9v ?

Yes sorry, that is correct. I had a brief dyslexic brainfart.

[SiC]

If you don't have heater wires on your existing one, you can use a heated iirc and just not connect those wires.

 

Heated lambda is to shorten the time till it operates, as sensor needs to be hot to work. Helps decrease the emissions from cold.

 

Given the age of the rx7 yours may or may not need a heater.

[tommotech]

The sampling rate of a multimeter will give an average reading, usually in the order of 500mV. An old analogue meter is required to see switching, where the needle will sweep. Avo meters are perfect but getting rare.

[Station]

I did try it with a multi meter - set to 20v I was getting 0.06 when it warmed up and idling at 800rpm.

3000rpm increased slightly to a limp wrested 0.13 max. Although just off cold it was reading 0.01, barely registering.

Shouldn't these be at 1.0 = 1v when set to 20v?

[Junkman]

If mounted to a F1 Stockcar for feeding back to the electronically controlled carburetter, you win races.

Hence BriSCA nixed them after I did.

[Station]

Idling voltage

 

 

Blat! And black sensor. This is my fireguard I put round the exhaust to stop it asphyxiating me in the garage.

 

[Dave_Q]

I think I'm right in saying that low voltage = lean. 

 

The state of the sensor would suggest that it's actually running very rich, which is what the ECU defaults to on most turbo cars if it doesn't get a sensible signal.

 

I would replace the sensor, generic one should be fine but try to get a reasonable brand not an ebay spesh.

[Station]

Thanks - I'm going to try Mazda, allegedly only 40 quid, or a Bosch one which are supposed to work fine.