hi all

think i might have a problematic sensor, not the one before the filter, where can i find it? its too cold to scrabble about under the car!

Engine code and year of car please.

Filter or catalyst?.

The sensors are on break apart Superseal type connectors fastened on a bracket at the rear and just right of the battery. You said 'filter' - do you mean the dpf and it's a diesel??

 

The top connector colored orange is EG temperature sensor 4, last in the chain in the first real section of exhaust.

The second one down colored black is EG temperature Sensor 1, the first and in the hottest place in the manifold pre-turbo.

The third connector down (black) has more than 2 pins and is the oxygen sensor. 

Hidden out of site on the back of the same bracket to fool you, is a fourth connector colored brown which is the dpf EG temperature sensor number 3 post turbo. Sometimes there may be EG sensor 2 but I've not seen one on a PD or CR engine yet.

 

What makes you confident that a sensor is dead?

 

 

 ok, sorry about delayed response, internet down for 3 days.

I was originally getting the exhaust inspection system lamp showing, vcds reported lambda sensor s1 oxygen sensor problem - see earlier posting, got a supposedly equivalent sensor, fitted it,, it worked for 2 engine restarts and the light showed again , but after about a minute. The info in the earlier posting indicated it all began to work properly after a covered mileage of 1500mls - ok thought that'll work then, but after the mileage no sign of rectification, took the probe out and returned it to the supplier for testing, no info currently. Put the original probe back in to gat the same response - 2 starts and the failure after 1 min on 3rd restart(i had previously cleaned the probe tip with a blow lamp etc). Car in for big service last wed, i asked the garage for info help etc and the guy thought it was probably the 2nd lambda probe hence my posting

2013   5L724Y(?)  or NFR  LA7W-8E1Z 103KW CFHC    Diesel

I now know a lot about these sensors. Not from what I've read on the internet, but what I've actually measured in 2 vag vehicles - an earlier diesel PD and later CR engine. There are massive holes to fall into with these sensors! First as you found is knowing what you are looking at and where it is. Second there are completely different types of (better!) sensors used on the older PD engine compared to the newer CR engine I tested. They may have used different sensors on other engines too, but I only have my own test evidence for 2. The later CR engine I tested used a sensor with a different curve characteristic for S1 compared to S2(if fitted) S3 and S4. You talk about Lambda probe and you need to be clear about what you mean. VAG use 2 wire temperature sensors which are NOT Lambda probes. Their Lambda probe is usually on a 4 wire or more connector and whilst it may (?) be able to measure temperature, it is a different device completely, measuring O2 levels.

 

if we are talking about 2 wire temperature only sensors you need to make sure what it is your are putting in and where, otherwise you will get fault lights lit. On the CR engine the S1 sensor in my opinion is most likely to fail, particularly with frequent regens because it is subjected to the highest extreme exhaust gas temperatures. I measured nearly 1000 deg.C. For this reason (although I'm sceptical) the sensor has a wider range compared to the others.  At room temperature it's resistance is over 2 Megohm whereas the S2-S4 sensors were the same but start at 25k ohm. Both EG sensors are NTC types which mean their resistance DECREASES non linearly as they get hot. Sensors used in PD engines are linear PTC type.

 

On CR engines the ECU has to work with these non-linear sensors and will have a fixed  algorithm or formula built in to match to the sensor characteristic it expects to be used. If you get the wrong part or put it in the wrong place you might get a fault shown, but worst case when your engine goes into regen mode, the high temperature it thinks is correct may be something else. These small parts are very important 'matched' devices and you could be 'playing with fire' literally if you don't know what you are doing.

 

Because the S1 sensor has a special characteristic, the ECU cannot detect it when it's cold. If you disconnect the S1 sensor with ignition on you get no fault light. Whereas you do for the S2-S4 sensors. The S1 sensor can only be verified as what the ECU expects when the engine has been running and the exhaust gas temperature has reduced the sensor resistance to a level the ECU can measure. Then if it's not about what it expects (compared to the other sensors?) a sensor fault will come on.

 

The manufacturer for most of these sensors would not give me information on their characteristic curve which I need for a project. I've just made a test box which allows me to find out what sensor curve the ECU expects on a CR engine. That's not the same as the sensor characteristic which can only be tested with a stable high temperature source such as a pottery kiln. However, if V.W/Skoda have done their job properly in the ECU, then my ECU curve should be close to the sensor characteristic.

 

Unless you know what you are doing, let the garage sort it and replace any sensors with genuine Skoda parts which correspond to your VIN number.

.

Edited 5 March, 20188 yr by voxmagna

ok, yes the vcds indicate s S1 the 5 wire unit, that was replaced with an equivalent to the vag skoda code, maybe thats the problem, i am just an enthusiastic ammertewer

; - D

sounds like skoda or my local vag outfit, non franchised garage it is then!

I did some outside the box tests on MY12 CR CFHC sensors this morning. S1 is definitely 2 wire and the middle black connector NOT the O2 sensor at the bottom. Getting to these sensors and even removing them is such a pain even when they aren't stuck well in which some are.  From my initial results, these thermistor types definitely aren't the type I would have chosen and it must be hard for them to hit the high dpf regen. temperatures repeatedly with any degree of consistency. It's a shame they went this route because the ECU A-D voltage conversion seems very good. Oh well, rubbish in =rubbish out! They must have been a few cents cheaper than using the PD engine platinum resistance linear type?

 

I bought a male/female pair of matching connectors so I can patch in easy. I prefer to check the temp. sensors at the connectors because you only have to lift the bonnet. However, those clever vag people use a tab moulding (as well as colors) to stop you plugging in to the wrong connector. My patch lead has to work with all the 2 pin sensors so I just filed slots at the right place in my connector shells and for testing they now mate with any of them.