Turbo Underboost Issue
Mr H called CSN Autos in Wymondham with a problem he couldn’t quite pin down.
His Land Rover Discovery Sport felt normal on flat roads, but the moment he reached an incline, the engine’s strength faded. The power dipped away, and a restricted performance warning message appeared before clearing itself once the road levelled out again.
Some trips were fine. Others raised questions he couldn’t ignore.
It didn’t take long for doubt to set in, and he wanted clarity on what was happening beneath the bonnet.
He needed answers — and a team capable of turning confusing symptoms into a diagnosis grounded in test results.
The next step was obvious: the vehicle needed a thorough evaluation, so we booked it in without delay.
Looking Into the First Signs of the Turbo Underboost Fault
Now that Mr H’s Land Rover Discovery Sport was with CSN Autos in Wymondham, the investigation could begin.
Our first task was to understand how the fault had appeared for him in day-to-day driving. We asked him to talk us through the reduced power on hills, how the vehicle reacted when the gradient increased, and whether the restricted performance warning message stayed on or cleared once the road levelled out again. These details matter; they shape how we recreate the issue during testing.
With this information gathered, we connected the Discovery Sport to our manufacturer diagnostic equipment and opened the relevant Land Rover technical data to ensure the evaluation followed the correct process. The initial scan returned a single stored code:
P0299-84 – Turbo/Supercharger Underboost
The turbocharger wasn’t producing the boost pressure the engine control module was commanding.
To understand what was happening under real-world conditions, we carried out a controlled road test around the hillier routes near Wymondham, recording serial data throughout. The symptoms reappeared exactly as Mr H described, allowing us to capture the fault under the right conditions. Back in the workshop, we analysed the logged data in detail.
The findings were consistent: the commanded boost pressure and actual boost pressure did not match. The engine control module was requesting more boost than the turbocharger was able to deliver. Under load, the system fell short of the target, which helped explain why the restricted performance warning message appeared during Mr H’s uphill driving.
Although this confirmed the boost pressure shortfall, it didn’t reveal the cause. A leak, a sensor issue, a problem with the actuator or a mechanical fault inside the turbocharger could all produce the same results. At this point, we moved on to a structured diagnostic evaluation.
Conducting a Full Diagnostic Evaluation for the Turbo Underboost Fault
With a confirmed shortfall between the commanded and actual boost pressures, the next stage was to determine the reason why. Each potential cause needed to be examined in a structured sequence, ruling out system components one by one.
These are the steps we took:
Step 1: Inspecting the Intake and Charge-Air System
- We began by examining the intake pipework, intercooler connections and associated hoses, checking for splits, loose joints or signs of disturbance.
- No issues were identified, so the intake and charge-air system were ruled out as contributing to the turbo underboost fault.
Step 2: Verifying System Integrity with a Pressurised Leak Test
- Using our pressurised leak detection equipment, we carried out a smoke test to confirm whether any boost pressure was escaping under load.
- The system held pressure without smoke loss, allowing us to eliminate pipework leaks and intercooler failures from the investigation.
Step 3: Testing the Boost Pressure Sensor and Wiring
- We assessed the boost pressure sensor using oscilloscope and circuit load testing methods, confirming correct supply, ground and signal integrity.
- All readings fell within specification, so neither the sensor nor its wiring was responsible for the boost pressure mismatch.
Step 4: Evaluating the Turbocharger Control Actuator
- Attention then turned to the electronic actuator, which controls how the turbocharger responds to engine load. We checked the actuator movement and response using an oscilloscope.
- The actuator and its wiring were in working order, ruling out a control-system issue.
Step 5: Mechanical Inspection of the Turbocharger
With leaks, sensors and control systems eliminated, the final focus was the turbocharger itself. A mechanical inspection provided the decisive answer: the turbocharger showed clear internal failure and was no longer capable of generating the commanded boost pressure under load.
The pattern matched with the serial data captured during the initial investigation, leaving the team in no doubt that this internal breakdown was the root cause of the turbo underboost fault.
Replacing the Failed Turbocharger
With the internal turbocharger failure confirmed, we contacted Mr H to explain the findings in detail. We talked him through the test sequence, how each potential cause had been ruled out, and why the turbocharger itself was the component responsible for the underboost fault. Once he approved the repair plan, we moved on to the replacement work.
Here’s what we carried out:
Turbocharger removal and preparation
- Gained access to the unit by removing the required intake pipework, heat shields and surrounding components.
- Inspected the turbocharger’s oil feed and return lines for any restriction or contamination that could affect the new unit.
Fitting the replacement turbocharger
- Removed the failed turbocharger and prepared the mounting surfaces to ensure a clean, accurate fit.
- Primed the lubrication system so the replacement turbocharger received oil immediately on first start-up.
- Installed an OEM-spec replacement turbocharger using the correct gaskets, seals and torque settings.
Reassembly and essential checks
- Reassembled the intake system and associated components, confirming all fixings, brackets and connections were correctly secured.
- Verified oil supply to the replacement turbocharger and checked for any air, oil or exhaust leaks after installation.
- Ensured all electrical connectors and vacuum or pressure hoses were correctly refitted.
- Cleared stored fault codes and prepared the vehicle for post-repair testing.
Final validation
- Carried out a controlled road test while logging serial data to assess boost pressure under load.
- The replacement turbocharger achieved the commanded boost pressure across the required load range.
- The restricted performance warning message did not return, and no additional faults were triggered during the assessment.
During the repair work, Mr H remained mobile using one of our courtesy cars, helping reduce disruption while the job was completed.
The Outcome: A Successful Repair
With the replacement work complete and all checks carried out, the Land Rover was ready for its final assessment. Out on the test route, the engine produced the level of power expected, with no hesitation on the climbs around Wymondham. The restricted performance warning message was no longer present, and the vehicle responded cleanly throughout the drive.
Back at the workshop, we reviewed the post-repair serial data and compared it to the readings gathered earlier in the investigation. The imbalance between commanded and actual boost pressure had been eliminated, and the turbocharger was now reaching the level the engine control module required. Everything pointed to the same conclusion: the internal turbocharger failure had been resolved, and the Discovery Sport was operating as it should.
Your Local Land Rover Specialists in Wymondham
If your Land Rover is losing power, showing warning messages or struggling on hills, it’s important to have it assessed by a team who understand these vehicles in detail. Issues like the turbo underboost fault on Mr H’s Land Rover Discovery Sport often develop gradually and become more noticeable on the roads around Wymondham and Norwich.
At CSN Autos, our technicians have the tools, technical information and diagnostic capability needed to trace Land Rover faults with accuracy. Whether you’re dealing with reduced power, inconsistent boost pressure, electrical concerns or warning messages, we can help.
By choosing us, you benefit from:
- Specialist diagnostics using manufacturer-level equipment
- Access to Land Rover technical information and test plans
- Evidence-led fault tracing carried out by experienced technicians
- Clear communication throughout the diagnostic and repair process
- A 12-month parts and labour guarantee
- Courtesy car available to keep you mobile while we work
Join the {{review-count}} drivers who rate us {{average-rating}} stars on Google and trust us to look after their vehicles with the same technical attention we apply in our workshop.
If your Land Rover is showing similar symptoms, contact us today on 01953 425983 and we’ll arrange a booking for a full diagnostic assessment.
