Turbo Underboost Fault
Mrs T called Ystrad Service Centre with a problem she couldn’t quite pinpoint.
Each time she drove up the hills around Ystrad Mynach and Caerphilly, her Land Rover hesitated. Power fell away just when she needed it most. And the restricted performance warning appeared on the dashboard, only to vanish again once the road levelled out.
Intermittent behaviour like this can leave any driver second-guessing their vehicle. One day, it felt normal. The next, something was clearly not right.
Before long, the doubt set in.
Mrs T wanted answers, and the uncertainty to end. She needed a team that could take inconsistent symptoms and turn them into a clear, evidence-backed diagnosis.
We knew exactly what was needed: the Land Rover required a thorough diagnostic evaluation, so we booked it in as soon as possible.
Identifying the Early Indicators of the Turbo Underboost Fault
With the vehicle in our Ystrad workshop, we began our assessment.
The first step was to understand exactly when and where the symptoms appeared. We asked Mrs T to explain what happened on the climbs through Ystrad Mynach and Caerphilly, how the vehicle responded under load and whether the restricted performance message stayed on or cleared once the engine was no longer working as hard. This helped us replicate the conditions during testing.
We then connected the vehicle to our manufacturer-level diagnostic tool and accessed the relevant Land Rover technical information to guide the assessment. The initial scan revealed one 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 normal driving conditions, we carried out a controlled road test using routes similar to those Mrs T had described. Throughout the drive, we logged serial data to record how the engine behaved under increased load. The hesitation she experienced reappeared exactly as described, giving us a clear sample of data for review.
Back in the workshop, we took a look at the data. A detailed analysis of the recorded information showed a noticeable imbalance. The commanded boost pressure and the actual boost pressure were not aligning when the vehicle was under load. The engine control module was requesting more boost than the turbocharger could supply, which explained the restricted performance message.
This confirmed a boost shortfall, but the root cause remained unknown. A range of issues can lead to this behaviour. It could be a leak, a sensor issue, an actuator problem or a mechanical fault within the turbocharger itself.
So the next stage was to move to a structured diagnostic evaluation to narrow it down accurately.
The Diagnostic Process That Pinpointed the Turbo Underboost Fault
With the symptoms reproduced and the P02999 fault code noted, the next stage of the investigation focused on identifying the exact cause. Each system that influences boost pressure was examined one step at a time.
These are the steps we took:
Step 1: Inspecting the Intake and Charge-Air System
- The intake pipework, intercooler joints and associated hoses were visually inspected for splits, loose fittings or disturbances that could allow boost pressure to escape.
- No issues 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 pressurised leak detection equipment, we introduced smoke into the intake and charge-air system to determine whether any boost pressure was escaping under load.
- The system held pressure correctly, confirming that the intercooler and pipework were intact.
Step 3: Testing the Boost Pressure Sensor and Wiring
- The boost pressure sensor was assessed using oscilloscope readings and circuit load testing to verify correct supply, ground and signal output.
- All readings were within specification, allowing us to eliminate both the sensor and its wiring from the investigation.
Step 4: Evaluating the Turbocharger Control Actuator
- We checked the turbocharger’s electronic actuator to ensure it responded correctly to commands from the engine control module as the load changed.
- Oscilloscope and wiring checks confirmed stable movement and consistent signals, ruling out the actuator and its control system as contributors to the fault.
Step 5: Mechanical Inspection of the Turbocharger
With other components eliminated, we examined the turbocharger directly. A mechanical inspection revealed clear internal deterioration, limiting the unit’s ability to supply the boost pressure demanded under load. This finding mirrored the serial data gathered earlier, confirming the turbocharger itself as the component responsible for the underboost fault.
Restoring Boost with the Turbocharger Replacement
With the source of the fault confirmed, we contacted Mrs T to explain the findings and outline the recommended repair. We talked her through the diagnostic steps taken, the parts we had ruled out and the evidence showing that the turbocharger was no longer able to reach the required boost pressure. Once she approved the work, we moved ahead with the replacement.
These are the steps we took:
Turbocharger removal and preparation
- Access to the turbocharger was gained by removing the necessary intake pipework, heat shields and surrounding components.
- The turbocharger’s oil feed and return lines were inspected for restrictions or contamination that could affect the new unit.
Installing the new turbocharger
- The failed turbocharger was removed, and the mounting surfaces were cleaned and prepared for accurate installation.
- The lubrication system was primed to ensure the new turbocharger received oil immediately during its first start-up.
- An OEM-spec replacement turbocharger was installed using the correct gaskets, seals and torque settings.
Reassembly and essential checks
- The intake system and associated components were refitted, ensuring all fittings and brackets were securely reinstated.
- Oil supply to the new turbocharger was verified, and checks were carried out for any air, oil or exhaust leaks.
- Electrical connectors, vacuum lines and pressure hoses were correctly reattached.
- The stored fault code was cleared, and the vehicle was prepared for its final assessment.
Final validation
- A controlled road test was completed with serial data logging to evaluate how the replacement turbocharger performed under load.
- The new unit achieved the commanded boost pressure consistently across the required operating range.
- No warning messages appeared during the drive, confirming that the fault had been successfully resolved.
The Outcome: Land Rover Ready to Climb Hills Again
The Land Rover performed smoothly during its final test drive, with stable power delivery on the climbs around Ystrad Mynach and Caerphilly. The hesitation Mrs T had noticed was no longer present, and no warning messages appeared. The engine responded well throughout the drive.
The turbocharger was now performing to the level required by the engine control module, confirming that the underboost fault had been resolved. Mrs T collected her Land Rover confident that the boost issue had been accurately identified and fully resolved. She was delighted with the thorough approach the team had taken and felt satisfied that the root cause of the problem had been dealt with.
Land Rover Specialists Supporting Drivers Across Ystrad Mynach and Cardiff
If your Land Rover is struggling with power loss, hesitation on hills or warning messages under load, it’s important to have it assessed by a team who understand these vehicles in detail.
At Ystrad Service Centre, we use manufacturer-level testing equipment and the correct Land Rover technical information to assess performance issues accurately. Whether your vehicle is showing a boost shortfall, displaying warning indicators or developing symptoms under load, our technicians are here to help.
By choosing us, you benefit from:
- Advanced testing using manufacturer-level tools
- Access to the correct Land Rover technical procedures
- Evidence-led fault tracing carried out by experienced technicians
- Clear communication throughout your vehicle’s assessment
- A 12-month parts and labour guarantee
- Need to stay mobile? Use our courtesy car.
With a {{average-rating}} star rating from {{review-count}} Land Rover owners, we’re proud to support drivers across Ystrad Mynach and Cardiff.
If your Land Rover isn’t performing as it should, call 01443 540460 and we’ll arrange a comprehensive evaluation.
