Two 3.0‑litre diesel engines from Land Rover – the older V6 (306DT) and the newer straight‑six (AJ20D6). One is proven but carries a catastrophic risk; the other is modern, refined, and more reliable. Here's our honest verdict.
The AJ20D6 is the better engine for most buyers – it's the modern straight‑six with no catastrophic bottom‑end flaws, delivering smoother power and better refinement. The 306DT is older, cheaper to buy, but its infamous crankshaft bearing failure risk makes it a gamble you might not want to take.
Full comparison, cost analysis, and a scenario‑based buying guide below.
| AJ20D6 (3.0 I6) | 306DT (3.0 V6) | |
|---|---|---|
| Layout | Straight‑six | V6 |
| 2025 UK Enquiries | [ESTIMATED] | 980 [EM-VERIFIED] |
| Typical Host Vehicles | Defender L663, Discovery L462, Range Rover L460 | Discovery 4 L319, RR L405, RR Sport L494 |
| Typical Vehicle Value | £35,000–£90,000 | £6,000–£40,000 |
| Recon Supply (306DT) | N/A (limited used supply) | £6,500–£9,500 |
| Defining Weakness | Minor cooling / electronics | Crankshaft bearing failure |
| Best For | Reliability, modern comfort, lower risk | Budget buyers, off‑road enthusiasts who accept risk |
The AJ20D6 is the clear winner for reliability and long‑term ownership. Its straight‑six design is inherently smoother and more durable, and it lacks the 306DT's catastrophic failure mode. The 306DT is a capable engine, but its crankshaft bearing issue is a genuine time bomb that can turn a bargain purchase into a financial disaster. In our data, the 306DT's 980 enquiries [EM-VERIFIED] reflect its age and the fact that many owners are now facing repair decisions, not that it's a superior engine. If you can afford a vehicle with the AJ20D6, it's worth the extra investment for the peace of mind alone.
The AJ20D6 is Land Rover's modern 3.0‑litre straight‑six diesel, introduced around 2020. It's part of the Ingenium family and features a modular design with a single turbo or twin‑turbo setup (D250, D300, D350). It's used in the latest Defender, Discovery, and Range Rover models. The straight‑six configuration is inherently balanced, delivering smooth, refined power with excellent torque delivery. Compared to the 306DT V6, the AJ20D6 is more fuel‑efficient and emits lower CO₂. Crucially, it does not share the 306DT's crankshaft bearing weakness – reports of major bottom‑end failures are rare [PATTERN DATA]. The main issues are minor: cooling system components and some electronics, all well within normal repair costs. Reconditioned units are still scarce, but the engine is young enough that most owners won't need a replacement. Full specifications and fitment details are on our AJ20D6 data page.
The 306DT is the well‑known 3.0‑litre V6 diesel used across Discovery 4, Range Rover L405, and Range Rover Sport L494 from 2010 to around 2020. It's a proven engine with good power and torque, but it's infamous for a catastrophic crankshaft bearing failure that typically occurs between 80,000 and 140,000 miles [PATTERN DATA]. This failure can destroy the engine, and replacement costs are high – £6,500–£9,500 for a reconditioned unit plus 15‑20 hours labour [EM-QUOTE]. On older, lower‑value vehicles (e.g., Discovery 4s under £10k), this can easily exceed the car's worth, pushing them into scrap territory. Despite this, the 306DT is still popular because it's widely available, parts are cheap, and many owners are willing to take the risk for the sake of a capable, affordable off‑roader. Full details on the 306DT Knowledge Centre and the 306DT Crankshaft Bearing Failure page.
The cost difference is driven primarily by purchase price and the probability of a major failure. The 306DT is far cheaper to buy – a Discovery 4 can be had for under £10k, while an AJ20D6‑equipped Defender or Discovery will cost £35k+. However, the 306DT's failure risk is real; many owners will face a £7k‑£10k bill within five years, which often exceeds the vehicle's residual value. The AJ20D6, being newer and more reliable, rarely needs an engine replacement, so you avoid that large outlay. Servicing costs are comparable, though the AJ20D6 may have slightly higher dealer labour rates due to newer tech. Over five years, the AJ20D6 almost always works out cheaper to own, despite the higher entry price. Our repair vs replace framework can help you decide if a 306DT failure is worth fixing.
Your only option is a 306DT‑equipped vehicle – typically Discovery 4 or early L405. Focus on post‑2012 cars with a full history and evidence of a recent engine rebuild or crankshaft work.
AJ20D6, without question. The peace of mind from a robust, modern engine is worth the premium. You'll avoid the constant worry of a catastrophic failure.
If you need a rugged, affordable off‑roader and can accept the risk, a well‑maintained 306DT can still be a great choice – just budget for the worst.
AJ20D6 – it's more likely to cover 150,000+ miles without major issues, whereas the 306DT becomes a ticking time bomb after 80k.
To be clear: the 306DT is not a bad engine – it's served Land Rover well. But the AJ20D6 is a significant step forward in engineering and reliability. If you can stretch to a vehicle with the newer engine, it's almost always the better long‑term decision. For a broader view, visit our Engine Failure Index 2026 or the Economics hub.
Yes, significantly. The AJ20D6 has no catastrophic bottom‑end failures, while the 306DT is prone to crankshaft bearing failure. The AJ20D6's reported issues are minor by comparison.
The 306DT is cheaper because reconditioned units are widely available at £6,500–£9,500. The AJ20D6 is newer, so used/recon units are scarce and likely more expensive.
AJ20D6: Defender L663, Discovery L462, Range Rover L460. 306DT: Discovery 4 L319, Range Rover L405, Range Rover Sport L494.
For most buyers, yes – the reliability, refinement, and lower risk of catastrophic failure make it a better long‑term investment. The 306DT only makes sense if you're on a strict budget and accept the gamble.
980+ EnquiriesReal 2025 UK data from actual quote requests (306DT).
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12 Vetted JLR SpecialistsExperienced with both engine families.
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