Sub O2 sensors are they needed ?

[Tonyd]

Hi Guys,

Done a search but nothing has come up.

Just in middle of wiring up the first of the LS400's , waiting for a hose so should have it running next weekend. Because the engines are pre 1995 we can run here (UK) without cats, and therefore there is an opportunity to ditch the second set of O2 sensors ( sub o2 sensors as they are called.)

Two questions. ( Long ones however.)

1. If we run without the sub O2 sensors will it buggar up the fuel maps in the ECU, ie will the engine run badly? I read the manual and it states that the second set of sensors gives feedback compensation and corrects the "skip amount" generated by the initial information from the main O2 sensors. If I read this correctly basically the sub sensors back up the main sensors and fine tune the AF ratio. If I ditch the sub sensors would I really notice a drop off in performance.

2. If we run without cats and use the sub O2 sensors will the ECU recognise that the cats are absent/ non functioning and default to emergency settings, ie run inefficiently. Will the absence of cats affect the ECU in any way?

Of course we will always run with pre cat sensors in situ so the ECU will be getting most of the info it needs to run closed loop.

Thoughts please.

Cheers,

Tony

 

[sideshow]

are they the sensors in the cat

they are usually just temp sensors which alert the ecu that the cat has melted or blocked

they send a signal to the ecu then the ecu turns on a warning light in the dash

i never hook them up

 

[Tonyd]

Thanks Sideshow, I was hoping that I didnt have to use them. They are the sensors immediately after the cat. They are described as being identical to the pre cat main O2 sensors, however the sub sensors are not heated. I'm not using the standard dash so I wont be bothered about error codes.

Cheers,

Tony

 

[MaxPower]

The rear sensors measure the efficiency of the cat convertors.

One function of a 3 way cat is storing oxygen when the AFR is lean and releasing oxygen when the AFR is rich - under 14.7:1 AFR there is no oxygen remaining in the exhaust gas.

The way closed loop works is by cycling the AFR rich then lean to give an average AFR of 14.7:1, so the front O2 sensors will show a voltage swing that goes between about 800mv and down to about 200mV during closed loop, the rate and bias is determined by the ECU code.

So based on that you can expect the rear O2 sensors to see a flat line 450mV or there-abouts during closed loop - as the ECU command a rich AFR the cat releases its stored O2 from the last lean cycle and vice versa to get an avergae value of exactly 14.7:1 after the cats. If the rear O2 sensors start to see varying voltages that are within around 50% (ECU dependant) of the front sensors then the cat is considered to be non functional or begining to fail as can no longer store or release the required oxygen and a fault code will be logged.

 

[Cobber]

Can you run without the front and rear sensors?

 

[Tonyd]

Based on the above I would say its ok to run without the second set of sub sensors as their only real function is to detect if a cat is on its way out and flag up a warning light, but I think its imperative to keep the front set to provide inputs to the ECU unless you're going to carbs and a simple aftermarket ignition system.

Cheers,

Tony

 

[MaxPower]

Without front O2 sensors the ECU will run in open loop only. Fuel economy will be affected and of course emissions.

I am not sure how the ECU reacts to not having the rear O2s in terms of if it still allows closed loop. I play more with GM ECM/PCMs rather than Toyota's so someone else may be able to answer that question.