What happens when a pre-cat Lambda/O2 sensor goes out?

[lswing]

Getting some intermittent CEL's from my Lambda's in the header/pre-turbo. Got one for the sensor in the DP's. These have shown up only after hard driving from what I can tell. They seem to be getting affected by the lean/hot conditions I suppose. I plan to fix these conditions with better tuning, but...

Question, if they really do go out, what affect does it have on the ecu and performance? Does it put the car in limp?

Thanks!

[4everRS]

There are no O2's before the turbo. There's an EGT on each side, but the upstream o2 sensors are right in front of the precats in the DP. They are the singular most important sensor with regards to proper fueling. It is very hard if not impossible to diagnose a fueling issue without these in working order.

For replacement, the "book" says you need to drop the motor. However, it can be done by lowering the rear of the engine slightly with a transverse bar. They're kind of a bitch. Also, they should be replaced in pairs. Each side should be done at the same time. I would even consider doing all 4

[lswing]

Thanks, my bad on confusing with the EGT's. I think we replaced all 4 last year, I'll have to take a look at some papers. Agreed on the fueling, and cannot wait to get on the wideband o2 when I'm on the dyno in a few weeks!

But will the engine go limp without them reading, once said fueling issues are resolved?

[Dmb408]

Perfect thread for me right now. I am looking at a car that has seemingly had the engine dropped three times over not so many mile for these pre cat sensors. I have heard this before so I wasn't totally surprised. But what kind of car is going to have it happen three times over 58K miles? Two times it shows up on the service records (Elsa) as "front exhaust" or something like that, but you can tell it is the cat and/or sensor because later for the same entry it says "cat" and/or "sensor." The other time, the service description looks a little different but the engine is still dropped.

On another note, dumb question, what is EGT? Something, gauge, something?

[lswing]

Exhaust Gas Temp sensor...in the manifold/header pre-turbo.

I've heard of those sensors going, maybe not that often, and a talented mech can swap them without engine removal as mentioned.

[lswing]

Ok, so my lamda's are still acting up, it was intermittent before, now this...

17584 - Bank1: O2 (Lambda) Correction Behind Catalyst
P1176 - 001 - Control Limit Reached - MIL ON

Only after hard driving it seems. I'm assuming this is the second one down the line? They have been replaced in the last year. I'm thinking it's the extra fuel I'm dumping into the system, maybe combined with the water meth, sending too rich a mixture out the pipes, therefore giving the Control Limit Reached error?

No performance issues that I can feel, all seems well, just fishing for some thoughts out of general interest.

[mdegracia]

Because most mechanics are really hacks and the multiple engine drops are liking the result of doing re-work on a car that always needs the engine dropped to perform most repairs.