Turbo Underboost Fault
Mrs T contacted South Coast Garage in Eastbourne after her Land Rover Discovery Sport began losing power in a way she couldn’t quite make sense of.
On level roads, the vehicle felt normal. But as soon as she reached an incline, the engine’s power faded. Acceleration weakened. A restricted performance warning message appeared, then cleared again once the gradient eased.
Some journeys were unaffected. Others, especially on the climbs toward Brighton, Lewes and Hailsham, raised real concern.
The inconsistency left Mrs T unsure how serious the problem might be, and she needed clarity on what the symptoms actually indicated.
She needed to understand why the engine felt flat under load. She needed to know what the warning message was telling her. And she needed a team capable of converting those symptoms into a diagnosis supported by real test data.
The next step was obvious. The vehicle required a structured workshop assessment, so we booked it in without delay.
Initial Evaluation of the Suspected Turbo Underboost Fault
With Mrs T’s Land Rover Discovery Sport now at South Coast Garage in Eastbourne, the first priority was to understand how the fault behaved during her journeys. We discussed when the reduction in power first appeared, how the vehicle responded on steeper climbs, and whether the restricted performance warning message stayed on or cleared once the road levelled. This information helped establish the driving conditions we needed to reproduce during testing.
With the background clarified, we connected the vehicle to our manufacturer-level diagnostic tool and accessed the relevant Land Rover technical data to guide the assessment. The initial scan revealed a single stored fault code:
P0299-84 – Turbo/Supercharger Underboost
The turbocharger was not reaching the level of boost pressure requested by the engine control module.
To see how the issue developed under load, we carried out a controlled road test using routes similar to those Mrs T described. Throughout the drive, we logged serial data to capture how the engine responded when placed under greater demand. The fault reappeared exactly as she had experienced it, providing the information needed for further analysis.
Back in the workshop, we reviewed the recorded serial data in detail. The results showed a clear mismatch between the commanded boost pressure and the actual boost pressure achieved. Under uphill load, the turbocharger was unable to meet the level the engine control module required, which aligned with the restricted performance warning message Mrs T had noticed.
Although this confirmed a boost pressure shortfall, it did not reveal the underlying cause. A leak, a sensor signal issue, a control-system problem or a mechanical fault inside the turbocharger could all be behind it. To find the precise reason behind the underboost, the next stage required a structured diagnostic evaluation.
Tracing the Turbo Underboost Fault Through a Structured Test Plan
With the boost shortfall and P0299 fault code confirmed during our initial evaluation, the next stage was to determine why Mrs T’s Land Rover Discovery Sport could not reach the boost pressure the engine control module was requesting. To ensure the findings were accurate, each area of the system with influence over boost pressure needed to be examined in a clear, structured sequence.
These are the steps we took:
Step 1: Inspecting the Intake and Charge-Air System
- We carried out a visual inspection of the intake pipework, intercooler joints and associated hoses, looking for any splits, loose fittings or signs of disturbance.
- No defects were identified, allowing us to eliminate leaks or restrictions in this section of the system.
Step 2: Verifying System Integrity with a Pressurised Leak Test
- Using pressurised leak detection equipment, we introduced smoke into the intake and charge-air system under controlled pressure.
- The system held pressure without smoke loss, confirming that the intercooler and pipework were correctly sealed.
Step 3: Testing the Boost Pressure Sensor and Wiring
- We tested the boost pressure sensor to confirm the accuracy of its supply, ground and signal circuits.
- All readings were within specification, ruling out the sensor and its wiring as reasons for the lack of boost.
Step 4: Evaluating the Turbocharger Control Actuator
- We reviewed the turbocharger’s electronic actuator to confirm that it adjusted the turbo correctly when the engine control module altered its demands under varying load.
- Oscilloscope monitoring and wiring checks showed consistent movement and stable control signals, allowing us to rule out the actuator and its associated circuitry as potential causes of the boost shortfall.
Step 5: Mechanical Inspection of the Turbocharger
With the intake system, sensors and actuator all eliminated, the final focus was the turbocharger itself. A mechanical inspection identified internal wear that restricted the unit’s ability to deliver the boost pressure demanded under load. This pattern aligned with the earlier serial data.
This confirmed that turbocharger failure was responsible for the underboost failure.
Carrying Out the Turbocharger Replacement
With the source of the underboost fault identified, we contacted Mrs T to explain the findings and outline the repair plan. We talked her through the diagnostic process, the areas ruled out during testing, and the evidence showing that the turbocharger could no longer deliver the required boost pressure under load. Once she approved the repair, we proceeded with the replacement work.
Turbocharger removal and preparation
- Access to the turbocharger was gained by removing the required intake pipework, heat shields and surrounding components.
- The turbocharger’s oil feed and return lines were checked for any restriction or contamination that could affect the new unit.
Installing the new turbocharger
- The failed turbocharger was removed, and the mounting faces were cleaned and prepared to ensure an accurate fit for the replacement unit.
- The oil system was primed to ensure the new turbocharger received fresh oil immediately during its first start-up.
- An OEM-spec replacement turbocharger was installed using the correct seals, gaskets and torque settings.
Reassembly and essential checks
- The intake system and associated components were reassembled, ensuring every fitting and bracket was reinstated securely.
- Oil supply to the new turbocharger was confirmed, and checks were carried out for any air, oil or exhaust leaks.
- All electrical connectors and vacuum or pressure hoses were correctly refitted.
- The stored fault code was cleared, and the vehicle was prepared for its road test.
Final validation
- A controlled road test was completed with serial data logging to evaluate how the replacement turbocharger performed under load.
- The new unit reached the commanded boost pressure consistently.
- The restricted performance warning message did not return, confirming the fault had been resolved.
The End Result: Turbo Underboost Fault Resolved
With the replacement turbocharger fitted and the necessary checks completed, the Land Rover was ready for its final assessment. On the test route, the engine had delivered its power consistently, with no hesitation on the climbs around Eastbourne, Brighton and Lewes. No warning messages appeared, and the vehicle maintained steady performance throughout the drive.
Once back in the workshop, we reviewed the serial data recorded after the repair and compared it with the information gathered during the earlier investigation. The previous shortfall between commanded and actual boost pressure had been resolved, confirming that the turbocharger was now reaching the level required by the engine control module.
The results provided a clear resolution to the issue, and Mrs T’s Land Rover was once again performing exactly as it should.
Trusted Land Rover Expertise for Drivers Across Eastbourne
If your Land Rover is losing power, hesitating on hills or displaying warning messages under load, it’s important to have the issue assessed by a team who understand these vehicles in detail. The underboost fault on Mrs T’s Land Rover, for example, became especially noticeable on the climbs around Eastbourne, Brighton and Lewes.
At South Coast Garage, our technicians work with manufacturer-level testing equipment and the appropriate Land Rover technical information to pinpoint performance issues accurately. Whether your vehicle is experiencing a boost shortfall, showing warning indicators or developing symptoms as engine load increases, we’re equipped to help.
By choosing us, you benefit from:
- Vehicle testing using manufacturer-level tooling and processes
- Access to the correct Land Rover technical data for accurate assessment
- Evidence-supported fault tracing carried out by experienced technicians
- Clear communication at every step of the way
- A 12-month parts and labour guarantee
- A courtesy car available to keep you mobile
With a {{average-rating}} star Google rating from {{review-count}} drivers, we’re proud to support Land Rover owners across Eastbourne and the surrounding areas.
If these symptoms sound familiar, call 01323 734473 and we’ll arrange a full diagnostic assessment for your Land Rover.
