When is a fault code false? suspension

[KBS]

When is a fault a false reading?
I have read through the many posts on this site and other Jaguar forums on issues to do with the air suspension but I find little which seems to relate to the problems (real or imaginary?) that I am facing.
In the past 12 months I have had three air suspension compressors (and relays) fitted to my X358 – 1 (the car’s original unit) reconditioned by myself and 2 brand new OE compressors.
The issue has been the intermittent recurrence of the amber ‘air suspension warning’ light though there is and has never been any apparent fault with the suspension itself – no sagging, dropping or raising, nor one side or corner being lower than the other. In fact the only evidence of said fault in the suspension is the warning light and the code behind it c2302 – levelling plausibility error.
Stopping the car for 15 -30 seconds and then restarting it switches off the warning light – until the next time that is. Not much of an issue one might think but my god it’s annoying; more so because it tends to happen during a journey of a reasonable length when cruising at a steady 50 – 70 mph, sometimes when on cruise control and at other times when in typical ‘town traffic’ conditions or proceeding a slow speed or stop start traffic It is not an issue at start up.
The vehicle has been looked after and been to two different Jag specialists for the replacement of the air suspension compressor units and relays
About 10 months ago the original ‘refurbished’ unit and relay which suffered intermittently with the same fault light problems and associated code had to be replaced by a brand new OE unit for others. Over the next few months the fault light came on intermittently about once every 2 months or so.
When the car went on for its annual service and MOT the codes behind the warning light came back as c2302 – levelling plausibility error - but the unit was only some 9 months old and had covered only 5000 odd miles since it was fitted, new. The specialist in that instance could find no reason for the fault indicated by the code.
About 2 weeks ago I took my vehicle along to the second Jag specialist for some other work on the car on the way to the specialist the fault light came on and kindly checked the code - sure enough c2302 – and in their view they believed that it was a compressor fault.
So whilst it was in for the other work I bought another new OE unit and relay and this they fitted at the same time.
To keep this posting as short as possible – the air suspension warning light came back on again on the way back from this specialist to my home and also couple of times in the next 2 days or so. The car went back and was again checked to confirm that there were no leaks anywhere in the air suspension system, nor any faults with the ride level sensors, air valves etc. In other words it appears that the air suspension system is ‘air tight’. However the code that was showing was again c2302 – levelling plausibility error. So the code would suggest that this week old newly fitted air suspension pump was faulty (the third in under 12 months).
On the way back from this visit the air suspension warning light came back on after I had travelled some 20 miles or so and as before stopping the vehicle and switching off the engine for 15-30 seconds caused the light to extinguish. Also, as always there seems to be no physical manifestation of the fault that I can see or sense whilst driving the car.
The way this is happening is leading me to question as to whether the fault (and the code c2302) is due to a levelling plausibility issue within the air suspension set up, or is in fact coming on and being brought about/ activated by something else entirely, possibly to do with the electrics.
I do not see that the old original pump and the two brand new OE air pumps can develop the same symptoms and supposedly the same code with the space of a few months, or in the latter the space of a few days. If I add to this the fact that both specialists found that the rest of the air suspension was leak free/air tight, coupled with the fact there are no physical symptoms of this fault that I have experienced, then logic would suggest that the actual fault is not with the air suspension/ levelling plausibility but that this reading is being produced by something else.
Either way, fitting a new air suspension compressor every few weeks becomes extremely costly and very irksome – almost as irksome as the damn light coming on when you’re least expecting it.
Any advice, experience of or help anyone could give me would be most appreciated, together as to how I might ascertain what the issue really is and of course the details of anybody (Jag specialist, auto electrical engineer, air suspension specialist, etc.) who might be able to get to the bottom of this and rectify this for me (location contingent of course).

[PaulGover]

My workshop manual says for C2302 "Levelling plausibility error - Carry out mechanical checks (air leaks, etc). Refer to the service bulletins."
That doesn't help much, and I don't have access to the service bulletins to see if they help.

If I understand correctly, plausibility error means it doesn't believe its senses - which in this case will mean either info from the ride level sensors doesn't agree with the ECM's view of the vehicle's stance, or the vehicle doesn't appear to move up and down sufficiently (or at all) in response to commands to raise or lower the suspension. At first sight, it doesn't appear it would involve the pump - that would probably produce some more direct messages about being out of pressure.

I'd investigate the ride level sensors for correct operation, their wiring for damage, for blocked, damaged or leaking pipes, and for sticking valves. OK, these have already been covered, but perhaps not enough to find an intermittent fault. Alternatively, there are yet more sensors that tell the ECM the car's position (front and rear vertical accelerometers, acceleration sensor).

Another possibility is some difference between the expected operation of the air springs and reality. Eg, a spring stuck at one level, so while the system can keep the car level (with that spring), it can't raise or lower the car.

You've been to a couple of Jaguar specialists. Were they dealers? If not, they'll presumably be using 3rd-party diagnostic tools such as ODB-II code readers, instead of the Jaguar Diagnostic tool (which can do exciting things like calibrate the suspension), so they might be worth a try (at yet more cost).

[J44EAG]

I agree with Paul. A good, logical diagnosis progression.

I would be looking at the two front height sensors attached to the lower drop arms. The OS unit doesn`t do much except look pretty. On the later cars, it is there purely to complete a circuit. On early cars it was functional. The NS sensor is the one to look at. Check it is still properly fixed to the drop arm and that it isn`t falling to bits. That is the active sensor and could well have age and mileage related issues. Check for good plug and socket connections. A tired sensor could indeed cause plausibility problems.

Mike

[KBS]

Paul/Mike thank you both for your responses - much appreciated. I take the point about the main dealer diagnostics and although more money it might be worth going that route to see if that solves the issue or pinpoints it a bit more clearly.

Have spoken with Arnott-Wabco's technical people for their thoughts as I don't believe that the issue is to do with the compressor [and unsurprisingly, they are inclined to agree, given that it is the third compressor in a year] so your approach seems to logically flow from that surmise

[njepson394]

Hi KBS,

Did you solve your problem in the end?