C4GP 1.6 BlueHDi first cat up/downstream temp sensors reversed is okay?

[I want my XM back]

A strange query, thoughts welcome.

I had put the explanatory how-we-got-here ramble then the question about aftermarket twin temperature sensors for the 1.6 BlueHDi. The ramble is really to share knowledge about leaking exhausts and removing the sensors so I have moved the punchline question to the top. I don't think this has been discussed, based on a search.

The 1.6 BlueHDi has two temperature sensors, common 4 pin connector, to measure before and after the first cat. I think 2014-2018 this was often 9808312080, that is the part on our 2015.

These sensors are not cheap, I opted for a new, aftermarket part - a lesser known brand. Only when I came to fit, did I realise that the two sensors are reversed on the new part. On our 9808312080, the top, upstream sensor is towards the rear of the connector as fitted above the alternator. Looking at images, Valeo and others appear correct. I called the manufacturer, who did speak to me as an end user which is good. I wondered whether there was confusion with the later 9812376480 from 2018, which does have short/long wires the other way round. Their engineering department confirmed upstream/downstream sensors could be reversed without issue, this was not important to the engine ECU. This seems odd to me. Diagbox shows the ECU does measure them separately (exhaust line 3) so as a minimum a basic fault will show reversed. The sensor manufacturer says the average of the sensors is taken, but why then have two sensors? Just have one. Surely two sensors are used to measure what is happening in the cat, or could have been used for that.

I could just send the new sensor pair back, I would have to pay return postage as the manufacturer says all is good. That would be cheaper than time spent me writing and you reading this but this reminds me of the 1994 purchase of XM track rod ends from Citroen Hammersmith. I said they should be handed, I was wrong apparently. In the day, you could talk to Slough and they said they are indeed handed and don't fit the wrong part. I think it is important to share the experience, particularly because these modern systems generate so many conflicting fault codes that we don't want to invite them.

Has anybody been here? I have ordered a second hand sensor pair, pictures of wiring match the original, will probably just fit that. If anyone knows how to repin the FCI/Sicma connector, I could have a go at that and keep the new one for spare after repinning. Shouldn't have to, though.

Chris


Even more ramble...might help some BlueHDi owners with holed first cat,

A year ago, we replaced the wife's perfectly good 2007 C4GP 1.6 diesel EGS (130k mile) with a 2015 C4GP 1.6 BlueHDi automatic (60k mile). A wasteful, and expensive, exercise driven by ULEZ. The new car was meant to have had an oil change, life etc got in the way, so only recently have I dropped the undertray to change the oil. Dear me.

An old Citroen check showed a corroded catalytic converter, repair under warranty rejected. So surely it had been repaired elsewhere? Or not.

Only with the tray down, did I find two broken jubilee clips and the remains of repair bandage on the first cat connection going backwards. Soot all over the top side of the undertray.

From zero knowledge a few weeks ago, I can now say the BlueHDi (well our 1.6) have an oxidation catalyst right at the engine, where our old 1.6 had the DPF. Exhaust turns 90 degrees under the engine, there's an adblue injector mounted forwards of the exhaust on the cat bend, firing urea straight into the flow of the exhaust towards the back of the car. The cat has a flange connection to a flexible that heads to a combined SCR (second cat) and DPF in one can under the car. Marc provided the diagram here viewtopic.php?t=81591

The first cat flange to the flexible corrodes where it is welded to the cat pipe. Truly awful quality, worst quality I have seen in 30+ years of Citroen ownership. The 2015 exhaust looks twice as old as our 2007 exhaust did. The rusty flange is so bad that you can buy an aftermarket repair flange from HJS in Germany. Typical degradation, like ours, shown at 20 seconds,

https://www.youtube.com/watch?v=DCUptGvaNwE

Our existing cat was 9805130480. The YouTube description for HJS repair kit 91211658 suggests it is for the later 9823467180 (ebay examples are 2017/2018), which it says replaced our 9805130480 and 9815574580. But the issue must also affect the early cats because ours was holed and there are other rough, early examples on ebay. All these generations appear in ebay listings, with holes, attempts at rewelding, completely missing the flange, or the flexi pipe welded direct with no flange.

My first ebay used cat purchase, still fitted with upstream+downstream temperature sensors (one part, shared connector) and adblue injector did not ship. The breaker offered no explanation. I came close to buying a new cat in desperation. They are about £1k list but two sellers have the new, PSA part for £600. In case the new quality was still just as bad, I sourced a used one from ebay, but without sensors and injector.

The existing cat lower temperature sensor finally moved after heat and penetrating oil. The top sensor resisted. I had bought a 14mm brake pipe spanner, not trusting the cheap sensor sockets with bad reviews. With hindsight, I should have poured penetrating oil into the top sensor (between flange nut and sensor body) and around its thread for days/weeks before I tackled the job. With the cat off, clock ticking, top sensor stuck, I cut the wires with the intention of soldering together. Even with a full 14mm socket and a 2 foot bar, more heat, more oil, the sensor did not shift. I gave up, had a cup of tea, realised I had nothing to lose and removed the sensor with more force than a very overtight wheel bolt. I don't think a split sensor socket would have worked, but I have since added a new VW T10401 socket to my collection (£30 ebay). It is 3/8" drive, well made but I think that would have snapped the drive. The top sensor was difficult to undo for two reasons - rusted thread, but also backing nut binding to the outside of the sensor so the conical faced sensor had to turn against the cat mounting face. Off the car, 24 hours soaking with penetrating oil between nut and sensor allowed them to move against each other. The top sensor tube was corroded within the nut, where the lower one was not. Hence my recommendation to soak before you start the job, gravity will help for the top sensor.

Soldering the sensor wires did not go well, due to being high temperature wire not designed to solder. The soldered connection has done several hundred miles back in the car, but I don't want to go into limp mode if the soldering fails. Hence the new sensors and all that has followed.

[GiveMeABreak]

I couldn't read all that but are you getting your sensors mixed up?

The DPF will have upstream and downstream pressure sensors.
There will be an exhaust gas temperature sensor
There will be a NOx sensor.

If you have your VIN I'll be able to clarify what exactly you do have.

[I want my XM back]

Thanks Marc, most of that was so folk did not repeat my mistakes - more complicated repair job than I expected. There are two temperature sensors fitted to the first cat (oxidation catalyst) in front of the engine, I am happy with what came off but worried there are aftermarket options that reverse the upstream/downstream sensors. VF7**************[VIN obfuscated, can be read by forum staff].

[GiveMeABreak]

So just for clarification:

(1) Is the temperature sensor to measure the exhaust gas temperature.

DPF
(5) are the Upstream and Downstream Particle Filter Pressure Sensors, connected to (2), the Differential Pressure Sensor.

(7) Is the NOx sensor for the Adblue system.

So there's only 1 temperature sensor.

The only thing I haven't shown here is the UREA (Adblue) injector.

[I want my XM back]

Thank you for your quick reply Marc. I would call that two sensors (pre/post) sharing one connector, but yes. Which appear separately in Lexia (I think), so they're not wired together electrically (series/parallel) in the ECU and measured as a pair. The ECU knows which is pre/post and the aftermarket manufacturer says they have OE samples wired both ways, I have seen no evidence of this on ebay listings where all used parts match my original. I am not convinced they are right and wondered whether this reversal has come up before. Catalysts not my subject at all, Google suggests a minimum temperature is important in diesel oxidation catalysts for "light off". Knowing which temperature reading is before and which after seems important. Perhaps best I just return the part I received.

[GiveMeABreak]

I'm not sure we're talking about the same thing. The DPF pressure sensors at (5) don't measure temperature they measure pressure before and after the particle filter. The particle filter differential pressure sensor measures the difference in pressure of the exhaust gases between upstream and downstream of the particle filter. They have nothing to do with the Catalytic Converter. They inform the engine ECU when the particle filter is blocked so a regeneration can take place.

The exhaust gas temperature sensor of which there is only one shown here, is just so the engine ECU knows how hot the temperature is so it can decide when to regenerate the filter (and relevant operations). The NOx sensor is intended to measure the overall volume of nitrogen oxides and the mixture of the exhaust gas downstream of the deNOx system.

[GiveMeABreak]

Here's a typical DPF sensor (in this case fitted to the DV5 engine) in case you are referring to this and the upstream and downstream sensors must be connected correctly as you can see:

• (1) Particle filter differential pressure sensor.
• "b" Pressure take-off downstream of the particle filter.
• "c" Pressure take-off upstream of the particle filter.
• "d" Piezo-resistive sensor.
• "e" Integral electronics.

The sensor is a passive sensor with integral electronics, supplied by the engine ECU.
The particle filter differential pressure sensor receives the pressure upstream of the particle filter on its upper face and the pressure downstream of the particle filter on its lower face.

The difference between the pressure upstream and the pressure downstream of the particle filter causes the resistance of the piezoresistor sensor to vary.
Depending on the resistance of the piezo-resistive sensor, the integrated electronics generate a code which is transmitted to the engine management ECU.
The particle filter differential pressure sensor is able to carry out self-diagnostics and signal any fault to the engine management ECU.

Key
"S" Particle filter differential pressure sensor digital output signal.
(bar) Difference in pressure measured between the pressure upstream and the pressure downstream of the particle filter in bars.
The particle filter differential pressure sensor output signal is proportional to the difference in pressure upstream and downstream of the particle filter.

Description	Values
Supply voltage	5 ± 0,25 volts
Maximum supply current	20 mA
Range of differential pressure measured	Differential pressure measured between 0 and 1 bar
Maximum pressure admissible without irreversibly damaging the operation of the sensor	Maximum pressure upstream: 2,4 bars
Maximum pressure admissible without irreversibly damaging the operation of the sensor	Maximum pressure downstream: 1,3 bars
Maximum pressure admissible without mechanically destroying the sensor	Pressure upstream: 3,3 bars
Maximum pressure admissible without mechanically destroying the sensor	Pressure downstream: 1,4 bars

[I want my XM back]

I do appreciate this Marc, I can't be explaining myself well. (1) in the diagram is one part number but two sensors. Two wires each, independent, Negative Temperature Coefficient sensors about 20kOhm at room temperature. They test as NTC when held between fingers. Total four wires, into a 4 pin FCI/Sicma connector retained on top of the alternator. One sensor is screwed into the top of the first cat, which was the DPF in that location before BlueHDi which explains why many people list the BlueHDi oxidation catalyst as a DPF. The second sensor is low down on that first cat, after (or during?) the cat doing its job. I need to connect Diagbox when hot, but the principle is upstream temperature rises steadily then downstream temperature kicks up suddenly as the cat "lights off". Maybe there is no differential on these sensors, maybe not in the right place for that.

"The exhaust gas temperature sensor of which there is only one shown here, is just so the engine ECU knows how hot the temperature is so it can decide when to regenerate the filter (and relevant operations)." There is an upstream and a downstream exhaust temp sensor for the first cat, maybe the aftermarket supplier is right and they just take the average of two temperature sensors to determine exhaust temp for regen. In my mind, two temp sensors meant they could check the first cat was working. I will check live Diagbox at the weekend.

[I want my XM back]

I need to look at live Diagbox and report back whether the two temperatures deviate. Just parking here this academic paper https://core.ac.uk/download/pdf/77955404.pdf page 60 (pdf 82) shows temperature before cat (dotted red) and after (dotted blue) which demonstrates the reaction of the catalyst. I am thinking that is why they have two temperature sensors, and reversal would be bad.

[RichardW]

That still the pressure sensor, Marc, the temp sensor has 2 ends on - had a look at our C4 this morning, and it does indeed have 2 temp probes. Seems to me that they will need to be the right way around or it will confuse the ECU!

[GiveMeABreak]

Oh Jesus, really having a bad parts day! Can't see the wood from the trees. I'll remove that last post so as not to add anymore confusion.

Yes, info is limited on this Richard. I have a much simpler system, as mine (DV5) doesn't have (1) shown below:

[I want my XM back]

Chaps,

I can confirm Diagbox readings

EDC17C60 Exhaust Line Information 3
Temperature of the exhaust gas upstream of the pre-catalyst
Temperature of the exhaust gas downstream of the pre-catalyst

both read -30C when the four pin connector is unplugged, so I am looking at the right readings in Diagbox

After a mile drive from cold, stopped car, upstream was 132C, downstream 136C ie basically the same.

After four miles more, stopped car, upstream was over 300C, downstream over 600C, falling once stationary and screenshot at 287C/576C.

Once hot, ie 287C/576C, particle filter regen status had gone from "not requested" to "requested", NOx sensor heating "not activated" to "activated". So it was due a regen, but I think this first cat works all the time, nothing to do with the DPF regen? I guess it only heats the NOx sensor once the system is up to temperature.

Either way, I will return the new sensor with reversed wiring. Difficult to get the supplier to pay the return postage if the manufacturer says it is acceptable. It might be, I can't recommend it. I assume naming the manufacturer is not the done thing.

Had someone said this was an old question and reversal is okay, I would have used the new sensor.

Thanks,

Chris