Td5 Injector Harness Explained: Symptoms, Causes & Replacement Guide

If your Land Rover Defender Td5 or Discovery 2 Td5 has started to misfire, cut out, or feel flat and lumpy, there’s a good chance the injector harness is the culprit. The Td5’s injector harness (also called the “injector loom”) is a small, inexpensive part that can cause big headaches when it starts to fail — but the fix is simple.

What is the injector harness?

The injector harness sits under the rocker cover and connects the five injectors to the engine wiring loom. One end plugs into each injector; the other end passes through the rocker cover and connects to the main loom via a round/oval plug. Part number AMR6103 is the correct harness for Td5 engines used in Defender (1999–2006) and Discovery 2 (1998–2004).

Why do Td5 injector harnesses fail?

In short: oil wicking and ageing seals.

Oil wicking (capillary action): Engine oil naturally migrates along the copper strands inside the harness. Over time it travels past the rocker cover connector and into the engine loom — sometimes as far as the ECU’s red plug.

Hardened O-rings/seals: Repeated heat cycles harden the small seals on the harness where it passes through the rocker cover. Once the seal isn’t perfect, oil movement accelerates.

Heat & vibration: Normal Td5 life — heat, vibration, and age — eventually degrades the harness insulation and connectors.

None of this means your engine is unhealthy; it’s a well-known Td5 quirk, and replacement is considered maintenance.

Classic symptoms of a failing injector harness

Intermittent misfire (often worse when hot)

Rough idle, hesitation, or cutting out

Flat performance / limp-home feel

Hard starting

Oil present in the ECU red plug or the round plug at the rocker cover

Fault codes related to injector balance or misfire (on a suitable diagnostic tool)

If you unplug the ECU red plug and see engine oil, the harness is almost certainly wicking.

Can’t I just clean it?

Cleaning the ECU plug and the external connectors with contact cleaner is a good first step — but it won’t stop the source. If the internal loom is wicking oil, it will return. The reliable cure is to replace the injector harness (AMR6103). Severely oil-soaked main looms can be cleaned over time, and in stubborn cases the engine loom may also need replacing — but most owners get great results by fitting the new injector harness and cleaning the plugs a few times over the next weeks.

Replacement overview (what your mechanic will do)

Remove the rocker cover and unplug the five injector connectors.

Unclip and withdraw the old harness, noting the routing.

Install the new AMR6103 harness, ensuring each injector connector clicks positively.

Replace the rocker cover gasket if it’s flattened or brittle (highly recommended while you’re in there).

Clean the external round plug at the rocker cover and the ECU red plug with proper electrical contact cleaner; wipe away any oil.

Refit the rocker cover, re-route the loom neatly, and road-test.

Re-check and re-clean the ECU plug after a few days/weeks — a little residual oil may continue to migrate out.

(Tip: Avoid brake cleaner on plastics and seals; use proper electrical contact cleaner.)

Preventative maintenance tips

Inspect at service time: Quick look at the rocker cover plug and ECU red plug.

Keep breathers clear: Excess crankcase pressure can encourage leaks/wicking.

Fix early: Replacing the harness as soon as symptoms appear prevents oil soaking further into the loom.

FAQs

Q: Will a new harness fix my misfire straight away?
Usually yes. If the main loom is heavily oil-soaked, you may need a couple of follow-up cleans of the ECU plug while the remaining oil purges.

Q: How often do they fail?
There’s no set interval — age, mileage and heat cycles matter. Many owners replace the harness once and enjoy years of trouble-free running.

Q: Do I need a new rocker cover gasket?
It’s sensible to fit a fresh gasket while the cover is off. It’s inexpensive and helps ensure a good seal.

Ready to sort your Td5?

We stock AMR6103 Injector Harness for Defender Td5 and Discovery 2 Td5 with fast UK dispatch. If you’re unsure whether your symptoms point to the loom, our team can help you confirm before you order.