Turbo Underboost Fault
Mrs T contacted Shires Garage in Langport when her Land Rover Discovery Sport began losing power in a way she couldn’t quite explain.
On flat roads, the vehicle behaved normally. But as soon as she reached an incline, the engine’s strength faded. Acceleration dropped away, and a restricted performance warning message appeared before clearing once the road levelled again.
Some journeys passed without incident. Others, like the climbs toward Taunton, made her question whether she could rely on the vehicle.
The inconsistency made it difficult for her to gauge the severity of the problem, and she needed a clear understanding of the underlying cause of the strange symptoms.
She needed to know why the vehicle felt weak under load. She needed to understand what the warning message indicated. And she needed a team capable of turning those symptoms into a diagnosis grounded in real test results.
The next step was straightforward. The vehicle needed a full assessment in the workshop, so we arranged a booking without delay.
Beginning the Diagnostic Assessment of the Turbo Underboost Fault
With Mrs T’s Land Rover Discovery Sport now at Shires Garage in Langport, we began by building a clearer understanding of how the fault presented itself on the road. We spoke with her about when the power loss first appeared, how the engine reacted on the climbs toward Taunton, and whether the restricted performance warning message stayed illuminated or cleared as the road levelled. This information shaped the conditions we needed to recreate during testing.
Once the background was established, we connected the vehicle to our manufacturer-level diagnostic tool and accessed the relevant Land Rover technical information to ensure each step of the assessment followed the correct procedure. The initial scan revealed a single stored fault code:
P0299-84 – Turbo/Supercharger Underboost
The turbocharger was not achieving the boost pressure that the engine control module was requesting.
To observe the issue under real driving load, we carried out a controlled road test on routes similar to those Mrs T had described. Throughout the drive, we logged serial data to capture how the engine responded when placed under higher demand. The symptoms reappeared exactly as she had experienced them, providing us with the data needed for further evaluation.
Back in the workshop, we reviewed the recorded information in detail. The findings showed a clear imbalance: commanded boost pressure and actual boost pressure were not matching. Under uphill load, the turbocharger fell short of the level the engine control module was requesting, which explained the restricted performance warning message Mrs T had noticed during her journeys.
Although this confirmed a boost pressure shortfall, it did not identify the underlying cause. A leak, an incorrect sensor signal, a control issue or a mechanical fault inside the turbocharger could all lead to the same behaviour. To determine the precise reason for the underboost, the next stage required a structured diagnostic evaluation.
The Detailed Assessment That Identified the Turbo Underboost Fault
With the boost shortfall and P0299 fault code confirmed, the next stage was to understand why Mrs T’s Land Rover Discovery Sport wasn’t reaching the level of boost required. To ensure nothing was overlooked, each potential cause was examined in a logical, structured order.
These are the steps we took:
Step 1: Inspecting the Intake and Charge-Air System
- We visually inspected the intake pipework, intercooler connections and associated hoses for splits, loose joints or disturbances that could allow boost pressure to escape.
- No faults were identified, allowing us to rule out leaks or restrictions in this part of the system.
Step 2: Verifying System Integrity with a Pressurised Leak Test
- Using our pressurised leak detection equipment, we introduced smoke into the system to confirm whether any boost pressure was escaping under load.
- The system held pressure without smoke loss, indicating the pipework and intercooler were functioning correctly.
Step 3: Testing the Boost Pressure Sensor and Wiring
- We checked the boost pressure sensor using oscilloscope readings and circuit load testing to verify the accuracy of its supply, ground and signal outputs.
- All values fell within specification, confirming that neither the sensor nor its wiring contributed to the boost shortfall.
Step 4: Evaluating the Turbocharger Control Actuator
- We assessed the turbocharger’s actuator to verify that it positioned the turbo correctly when the engine control module adjusted demand.
- Oscilloscope monitoring and wiring checks showed consistent movement and stable signals, allowing us to rule out both the actuator and its control circuit as contributing factors.
Step 5: Mechanical Inspection of the Turbocharger
With leaks, sensors and control components eliminated, we turned to the turbocharger itself. A mechanical inspection showed clear signs of internal deterioration, preventing the unit from achieving the boost pressure requested under load. This confirmed the turbocharger as the source of the underboost fault.
The Work Required to Install the Replacement Turbocharger
The next step was to contact Mrs T to explain the findings and outline the proposed repair. We talked her through the diagnostic process, the components ruled out during testing, and the evidence showing the turbocharger itself was faulty. Once she approved the work, we proceeded with the replacement.
Here’s the steps taken:
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 inspected for any signs of restriction or contamination that might affect the new unit.
Installing the new turbocharger
- The failed turbocharger was removed, and the mounting faces were prepared to ensure a clean, accurate fit for the replacement.
- The lubrication system was primed so the new turbocharger would receive oil immediately during the 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 all associated components were reassembled, ensuring every fixing and bracket was securely reinstated.
- Oil supply to the new turbocharger was verified, and checks were completed 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 post-repair testing.
Final validation
- A controlled road test was carried out with serial data logging to observe how the replacement turbocharger responded under load.
- The new unit reached the commanded boost pressure across the necessary operating range.
- The restricted performance warning message did not return, confirming the fault had been resolved.
The Result: A Land Rover Performing as It Should
With the replacement turbocharger fitted and all checks complete, the Discovery Sport was ready for its final assessment. Out on the test route, the engine delivered its power smoothly, with no hesitation on the climbs around Langport and Taunton. No warning messages appeared during the drive, and the vehicle responded consistently across the full route.
Back at the workshop, we reviewed the serial data captured after the repair and compared it with the information gathered earlier in the investigation. The previous shortfall between commanded and actual boost pressure had been resolved, confirming that the turbocharger was now achieving the level required by the engine control module.
Everything pointed to the same conclusion: the fault had been addressed, and Mrs T’s Land Rover was once again performing as it should.
Your Local Land Rover Specialists in Langport
If your Land Rover feels low on power, hesitates on hills or displays warning messages under load, it’s important to have it checked by a team who understand these vehicles properly. Problems like the turbo underboost fault on Mrs T’s Discovery Sport often become more noticeable on the climbs around Langport and Taunton.
At Shires Garage, our technicians carry out testing using manufacturer-level equipment and the appropriate Land Rover technical data. Whether your vehicle is experiencing a boost shortfall, showing warning indications or developing more complex symptoms, we have the tools and expertise to help.
By choosing us, you benefit from:
- Testing with the same level of equipment used by main dealers
- Access to the correct Land Rover technical guidance and procedures
- A structured, evidence-led approach to isolating faults
- Clear communication throughout the diagnostic and repair process
- A 12-month parts and labour guarantee
- A courtesy car to keep you on the move
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If these symptoms sound familiar, call 01458 250570 and we’ll arrange a full diagnostic evaluation.
