Turbo Underboost Fault

Mrs T contacted Smith’s of London in Croydon after her Land Rover Discovery Sport began showing symptoms she couldn’t fully understand.

On level roads, the vehicle felt normal. But the moment she reached an incline, the power dropped away. Acceleration softened, and a restricted performance warning message appeared before clearing once the gradient eased.

Some journeys passed without any issues. Others left her concerned about how the vehicle would cope on steeper roads through Croydon, Beckenham and Coulsdon.

The inconsistency made it difficult for Mrs T to understand what the symptoms were pointing to, and she needed clarity on whether the fault was minor or something more significant.

She needed to know why the engine wasn’t responding under load. She needed to understand the meaning behind the warning message. And she needed a team capable of turning those symptoms into a diagnosis.

The logical next step was clear. The vehicle required a structured assessment, so we booked it in without delay.

Launching the Initial Investigation into the Turbo Underboost Fault

With Mrs T’s Land Rover Discovery Sport now at Smith’s of London in Croydon, we began by clarifying how the fault presented itself during her day-to-day driving. We spoke with her about when the loss of power first appeared, how the vehicle behaved on the climbs through Croydon, Beckenham and Coulsdon, and whether the restricted performance warning message stayed on or cleared once the gradient eased. These details helped define the conditions we needed to recreate during testing.

Once we had that information, we connected the Discovery Sport to our manufacturer-level diagnostic equipment and accessed the relevant Land Rover technical information to guide the assessment. The initial scan returned a single stored fault code:

P0299-84 – Turbo/Supercharger Underboost
The turbocharger was not achieving the boost pressure requested by the engine control module.

To observe the issue in real-world driving conditions, we performed a controlled road test along routes similar to those Mrs T described. Throughout the drive, we logged serial data to monitor how the engine responded when placed under greater load. The symptoms reappeared exactly as she had experienced them, allowing us to capture a complete dataset for further review.

Back at the workshop, we analysed the recorded information in detail. The data showed a clear shortfall: the commanded boost pressure and the actual boost pressure did not align. Under uphill load, the turbocharger was unable to reach the level the engine control module was requesting, which explained the restricted performance warning message that concerned Mrs T during her journeys.

While this confirmed a shortfall of boost pressure, it did not identify the root cause. A leak, a sensor signal issue, a control-system fault or a mechanical problem within the turbocharger could each cause the same symptoms. 

To pinpoint the exact reason, the next stage required a structured diagnostic evaluation.

A Structured Diagnostic Process to Confirm the Turbo Underboost Fault

With the boost shortfall confirmed during the initial investigation, the next objective was to determine why Mrs T’s Land Rover Discovery Sport could not reach the level of boost the engine control module required. To achieve an accurate outcome, each system that could influence boost pressure needed to be evaluated in a structured and logical order.

These are the steps we took:

Step 1: Inspecting the Intake and Charge-Air System

• We examined the intake pipework, intercooler connections and associated hoses for any damage, loose fittings or disturbances that could allow boost pressure to escape.
• No issues were identified, enabling us to rule out leaks or restrictions within 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 as expected, confirming that the intercooler and pipework were intact.

Step 3: Testing the Boost Pressure Sensor and Wiring

• We assessed the boost pressure sensor using an oscilloscope and circuit load testing equipment to check the accuracy of its supply, ground and signal circuits.
• All values met the expected parameters, meaning the sensor and its wiring could be eliminated from the investigation.

Step 4: Evaluating the Turbocharger Control Actuator

• The turbocharger’s electronic actuator was tested to ensure it positioned the turbo correctly in response to commands from the engine control module.
• Oscilloscope analysis and wiring checks showed correct operation, allowing us to rule out the actuator and its control circuit as contributing factors.

Step 5: Mechanical Inspection of the Turbocharger

With leaks, sensor issues and actuator faults eliminated, attention turned to the turbocharger itself. A mechanical inspection showed clear internal deterioration, preventing the unit from reaching the commanded boost pressure under load. This finding aligned with the serial data gathered earlier and confirmed the turbocharger as the source of the underboost fault.

The Repair Process: Installing a New Turbocharger

With the cause of the underboost fault confirmed, we contacted Mrs T to explain the findings and outline the recommended repair. We talked her through the diagnostic steps completed, the components eliminated during testing, and the evidence showing the turbocharger could no longer reach the commanded boost pressure under load. Once she approved the work, we began the replacement.

Here’s what we carried out:

Turbocharger removal and preparation

• We removed the required intake pipework, heat shields and surrounding components to gain access to the turbocharger assembly.
• The turbocharger’s oil feed and return lines were inspected for any signs of restriction or contamination.

Installing the new turbocharger

• The failed turbocharger was removed, and the mounting surfaces were prepared to ensure a clean, accurate fit for the replacement unit.
• The lubrication system was primed so the new turbocharger would receive oil immediately during the first start-up.
• An OEM-spec replacement turbocharger was fitted using the correct seals, gaskets and torque settings.

Reassembly and essential checks

• The intake system and associated components were refitted, ensuring every connection and bracket was reinstated securely.
• 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 reattached.
• The stored fault code was cleared, and the vehicle was prepared for its post-repair assessment.

Final validation

• A controlled road test was carried out with serial data logging to monitor how the replacement turbocharger performed under load.
• The new unit achieved the commanded boost pressure consistently across the required operating range.
• The restricted performance warning message did not return, confirming the repair had addressed the issue.

How the Land Rover Performed After the Repair

With the replacement turbocharger installed and all workshop checks completed, the Discovery Sport was ready for its post-repair assessment. Out on the test route, the engine responded well. No warning messages reappeared, and the vehicle maintained consistent performance throughout the drive.

Back in the workshop, we reviewed the serial data recorded after the repair and compared it with the figures gathered earlier in the investigation. The imbalance between commanded and actual boost pressure had gone, showing that the turbocharger was now meeting the level requested by the engine control module.

The findings confirmed that the fault had been successfully resolved, and Mrs T’s Land Rover was operating as it should once again.

Land Rover Support You Can Rely On in Croydon

If your Land Rover is losing power, hesitating on inclines or displaying warning messages under load, it’s important to have it assessed by a team who understand these vehicles thoroughly. Issues like the turbo underboost fault on Mrs T’s Discovery Sport often become more obvious on the climbs through Croydon, Beckenham and Coulsdon.

At Smith’s of London, our technicians work with manufacturer-level equipment and the correct Land Rover technical information to investigate performance faults with precision. Whether your vehicle is showing a boost shortfall, presenting warning messages or developing symptoms under higher demand, we have the capability to help.

By choosing us, you benefit from:

• Specialist testing carried out using manufacturer-level tools
• Access to the Land Rover technical data needed for accurate fault assessment
• A structured, evidence-led approach to identifying the root cause
• Clear communication from start to finish
• A 12-month parts and labour guarantee

Land Rover owners rate us {{average-rating}} stars from {{review-count}} reviews — a reflection of the care we put into every assessment.

If your Land Rover’s performance has changed, call 020 8674 3377 and we’ll organise a comprehensive assessment for you.

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