P0299 Turbo Underboost Fault: The Diagnostic Process Behind a Land Rover’s Power Loss

Mrs T contacted Roade Main Garage in Northampton after her Land Rover Discovery Sport began behaving unpredictably.

On level roads, the vehicle felt normal. But the moment she reached an incline, the power faded. Acceleration weakened. A ‘restricted performance’ warning message appeared, then cleared again once the road levelled.

Some journeys were unaffected. Others raised concern.

The inconsistency made it difficult for Mrs T to judge how serious the problem was, and she needed clarity on what the symptoms were pointing to.

She needed to understand why the engine felt so flat. She needed to know what the warning message meant. And she needed a team capable of turning those symptoms into a diagnosis built on evidence rather than speculation.

There was only one sensible next step. The vehicle needed a thorough assessment, so we booked it in without delay.

The Initial Assessment of the Turbo Underboost Fault

With Mrs T’s Land Rover Discovery Sport now at Roade Main Garage in Northampton, we began by building a clearer picture of how the fault appeared during her journeys. We asked her when the loss of power first became noticeable, how the vehicle behaved on hills, and how long the restricted performance warning message stayed on for.

With that background established, we connected the Discovery Sport to our manufacturer diagnostic tool and accessed the relevant Land Rover technical information to ensure the assessment followed the correct procedure. The scan presented a single stored fault code:

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

To observe how the issue developed under load, we carried out a controlled road test using routes similar to those Mrs T described. There we logged serial data to monitor how the engine responded when placed under higher demand. The symptoms reappeared exactly as she had experienced them, allowing us to return to the workshop with a full record of the fault.

A detailed review of the recorded data confirmed a clear mismatch: the commanded boost pressure and actual boost pressure were not aligning. Under uphill load, the engine control module was requesting more boost than the turbocharger could generate, which explained why the restricted performance warning message appeared during Mrs T’s journeys.

While this confirmed a boost pressure shortfall, it did not reveal the underlying cause. An intake leak, a sensor signal issue, a control problem or a mechanical fault inside the turbocharger could each lead to the same pattern, so the next stage required a structured diagnostic evaluation.

How We Examined Each System to Trace the Turbo Underboost Fault

With the boost shortfall confirmed during the initial assessment, the next step was to establish why the turbocharger on Mrs T’s Discovery Sport was unable to reach the level of boost the engine control module required. Each potential cause needed to be examined in a structured sequence to ensure the findings were accurate and evidence-led.

These are the steps we took:

Step 1: Checking the Intake and Charge-Air System

• We carried out a visual inspection of the intake pipework, intercooler joints and associated hoses, checking for splits or loose fittings that could allow boost pressure to escape.
• No defects were found, 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 intake and charge-air system under controlled pressure.
• The system held pressure without smoke loss, confirming there were no leaks in the pipework or intercooler assembly.

Step 3: Testing the Boost Pressure Sensor and Wiring

• We assessed the boost pressure sensor to verify correct supply, ground and signal integrity.
• All readings fell within the expected range. This ruled out the sensor and its wiring as the issue.

Step 4: Evaluating the Turbocharger Control Actuator

• The turbocharger’s electronic actuator was tested to ensure it responded correctly to commands from the engine control module across the relevant load range.
• The checks confirmed normal operation, so the actuator and its control circuit were eliminated from the investigation.

Step 5: Inspecting the Turbocharger

With leaks, sensors and control components ruled out, the focus moved to the turbocharger itself. An inspection revealed internal failure, meaning the unit could no longer operate as intended. This finding aligned with the serial data gathered earlier. 

With that, the diagnosis was confirmed. The root cause of the underboost fault was internal turbocharger failure.

How We Completed the Turbocharger Replacement

With the cause of the underboost fault confirmed, we contacted Mrs T to explain the findings and outline the repair plan. We talked her through each stage of the diagnostic process, the components eliminated during testing, and the evidence showing the turbocharger could no longer meet the commanded boost pressure under load. Once she approved the work, we moved ahead with the replacement.

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 checked for signs of restriction or contamination that could affect the new unit.

Installing the new turbocharger

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

Reassembly and essential checks

• The intake system and associated components were reassembled, ensuring every connection and bracket was refitted securely.
• Oil supply to the new turbocharger was verified, and checks were carried out for any air, oil or exhaust leaks.
• All electrical connectors and vacuum or pressure hoses were correctly reinstated.
• The stored fault code was cleared, and the vehicle prepared for post-repair testing.

Road test

• We carried out a controlled road test while logging serial data to monitor how the replacement turbocharger responded under load.
• The new unit achieved the commanded boost pressure consistently across the required operating range.
• The restricted performance warning message didn’t return, confirming the repair had been a success.

The Outcome: Land Rover’s Performance Restored

With the repair complete, Mrs T’s Land Rover was back to performing the way it should. The loss of power had gone, the engine pulled cleanly when she accelerated, and the restricted performance warning no longer appeared. Because the fault had been identified correctly from the start, everything was resolved during the first visit.

For Mrs T, it meant confidence that the problem had been found, fixed and confirmed using the right diagnostic process. And while the work was being carried out, she stayed mobile with one of our courtesy car options, keeping her plans for the week on track.

Her Land Rover left Roade Main Garage running as it should once again, with the turbocharger operating correctly and no further signs of the underboost issue that brought her to us.

When Your Land Rover Needs Expert Attention, We’re Here to Help

If your Land Rover is showing reduced power, hesitation on hills or warning messages under load, it’s important to have it assessed by a team who understand these vehicles properly. Issues like the turbo underboost fault on Mrs T’s Discovery Sport can become more noticeable on the climbs around Northampton and Milton Keynes.

At Roade Main Garage, our technicians use manufacturer-level diagnostic tools and the correct Land Rover technical information to pinpoint performance issues with accuracy. Whether your vehicle is showing a boost shortfall, displaying warning messages or developing more complex symptoms, we’re here to help.

By choosing us, you benefit from:

• Evidence-led fault tracing carried out by experienced technicians
• Specialist diagnostics
• Clear communication throughout your repair
• Access to Land Rover technical data and guided test routines
• 12-month parts and labour guarantee
• Courtesy car options available

Join the {{review-count}} drivers who rate us {{average-rating}} stars on Google and trust our team to look after their Land Rovers every day.

If your Land Rover is showing similar symptoms, contact us on 01604 862262 to arrange a full diagnostic assessment.