06/06/2026
“Nobody could fix it.”
That was the story behind this 2013 Skoda Superb 2.0 TDI CR (CFFB).
For almost two years this vehicle had effectively become a driveway ornament. The last MOT was carried out on 7th October 2023 at 200,132 miles. Meaning the majority of its recent life has been spent parked up while fault after fault was chased without ever identifying the true root cause.
What initially appeared to be a straightforward P0651 fault quickly turned into one of the worst electrical failures I’ve ever seen on a VAG common rail diesel platform.
According to Ross-Tech, fault code P0651 / 17035 relates to “Sensor Reference Voltage B Circuit Open”. In simple terms, the ECU generates a stable 5V reference supply which is shared across multiple engine management sensors. When that reference voltage is lost, shorted or corrupted, the ECU begins receiving implausible information from numerous systems simultaneously.
On the CFFB engine, that 5V network is responsible for critical components including the boost pressure sensor, intake air temperature sensor, coolant temperature sensors, mass air flow sensor, turbocharger actuator position sensor, boost pressure control solenoid, fuel pressure regulation components, EGR system and numerous other engine management inputs.
The result?
A vehicle that appears to have dozens of unrelated faults when in reality they’re all symptoms of the same underlying problem.
The vehicle presented with coolant control faults, boost control faults, injector circuit faults, sensor reference voltage faults, immobiliser faults, communication faults, cooling fan issues and numerous implausible signal errors.
Every time one fault looked solved, another would appear.
After extensive diagnostics it became clear why.
The injector loom was burnt out.
The N75 boost control valve was burnt out.
Coolant control sensor wiring had failed.
Multiple sections of the loom had previously been cut, repaired, cut again and repaired again.
Several branches contained old splice repairs.
Multiple connectors were missing completely.
Loom routing was incorrect.
Sections of wiring had clearly been modified repeatedly over many years.
The more we stripped back, the worse it became.
At this point it was obvious that repairing individual wires was no longer a sensible option. The entire foundation of the engine management system had become unreliable.
The biggest discovery came when the ECU itself was removed.
The original ECU had already been opened previously. The casing seal had been disturbed and, unbelievably, a live wire had been run from inside the ECU directly towards the battery.
Not factory.
Not professional.
Not something that should ever be present inside a Bosch EDC17 control unit.
At that moment the original ECU could no longer be trusted.
Had voltage travelled the wrong way through that improvised wiring there was every possibility of damaging internal voltage regulation and grounding circuits inside the ECU itself.
At this stage I reached out for help.
Mechanics said call an auto electrician.
Auto electricians said call a mechanic.
Nobody wanted the responsibility of a vehicle that looked like it had spent years being repaired by different people with different theories.
One person did.
Massive credit goes to Florin Serban at Revv Mobile Auto
Instead of guessing.
Instead of throwing parts at it.
Instead of replacing another sensor and hoping for the best.
He approached it logically.
I had a complete donor vehicle sat in my yard that I will never get round to fixing and both the engine wiring harness and donor ECU were obtained.
The donor loom was installed because the original harness had suffered extensive damage throughout the 5V reference network, injector circuits and multiple engine management branches.
Trying to repair individual conductors would simply have meant chasing faults forever.
The complete engine loom was removed.
The ECU connector assembly was replaced.
Damaged wiring was eliminated.
Sensors were replaced where required.
Loom routing was corrected and returned to factory specification.
The donor ECU was then bench connected using AutoTuner.
A complete backup procedure was carried out and the replacement ECU prepared for installation.
Because the donor ECU carried a different immobiliser identity to the vehicle, immobiliser adaptation was not a viable option. The replacement ECU was therefore professionally configured with an immobiliser-off solution and installed alongside the donor loom.
Effectively the vehicle received an entirely new electrical foundation.
A new loom.
A new ECU.
Correct wiring.
Correct routing.
Correct diagnostics.
The transformation was immediate.
The cooling fans stopped behaving erratically.
Dashboard warnings disappeared.
Communication faults vanished.
Sensor values became stable.
Live data became believable.
The vehicle started, idled and ran exactly as it should.
A full Autel diagnostic scan now returns no stored faults other than a single known boost pressure control circuit fault. Unlike before, this fault is fully understood and isolated to a missing component that was not present on the donor vehicle because it had previously been used elsewhere.
Most importantly, the giant web of electrical faults affecting almost every engine management system has been completely eliminated.
To complete the project, the replacement ECU was calibrated with:
✔ Stage 1 Software
✔ IMMO OFF
✔ EGR
✔ DPF
✔ Readiness Calibration
✔ Soft Rev Limiter Removal
✔ Speed Limiter Removal
Performance increased from:
140bhp / 320Nm
180bhp / 400Nm
But the real victory wasn’t the extra power.
The real victory was bringing a vehicle back from the dead that many people had effectively written off.
This wasn’t a tuning job.
It wasn’t an emissions solution.
It wasn’t simply replacing an ECU.
It was a complete electrical rescue mission involving a donor loom, donor ECU, injector circuit repairs, sensor network restoration and the elimination of years of previous failed repairs.
Sometimes fault codes don’t tell you which sensor is broken.
Sometimes they’re telling you the entire electrical ecosystem underneath the engine management system has collapsed.
In this case it wasn’t one sensor.
It wasn’t one wire.
It wasn’t one connector.
It was a combination of burnt injector wiring, failed sensor circuits, missing connectors, years of historic repairs, a compromised ECU and a destroyed 5V reference network all working together to create one enormous fault.
Two years off the road.
Less than 24 hours to repair properly once the true root cause was identified.
Huge thanks again to Florin Serban at Revv Mobile Auto for taking on a job that nobody else wanted and proving once again that proper diagnostics will always beat guesswork.
Diagnostics first. Results that last.
Precision Remaps UK
Diesel Diagnostics & ECU Calibration Specialists
5★ Rated Service | Nationwide Mobile Tuning
[email protected]
precisionremapsuk.com
07356 062066 WhatsApp & SMS
