Land Rover Discovery 5 Transmission Fault P1707-77 and U0402: Causes, Diagnosis, and Fixes

ByWyatt Brooks

What These Two Codes Are Actually Telling You

P1707-77 and U0402(00) appearing together on a 2018 Land Rover Discovery 5 point to one specific system: the transmission park lock circuit. P1707-77 means the ZF 8HP gearbox cannot confirm whether the park pawl is engaged or released — the park/neutral position circuit has detected a short to ground or an open circuit. U0402 is the downstream result: other control modules on the CAN bus are receiving invalid or implausible data from the Transmission Control Module (TCM), so they log their own fault separately.

This combination after a sudden stop-dead event is almost never catastrophic internal gearbox damage. It’s an electrical or electromechanical fault in the park lock chain — and the first checks are quick and free.

The ZF 8HP Gearbox: Everything Is Electronic

The Discovery 5 3.0 diesel uses the ZF 8HP automatic transmission. There is no mechanical cable connecting the rotary gear selector to the gearbox. Everything is electronic — CAN bus signals instructing the mechatronic unit which gear to engage. The park lock itself is actuated by a solenoid inside the valve block. When the driver shifts out of Park, the TCM commands the solenoid to release the pawl, and a sensor confirms the move happened.

If that confirmation never arrives — because the solenoid circuit is shorted, open, or the sensor is sending bad data — the TCM throws P1707-77. And because the TCM is now broadcasting unreliable park lock status across the network, other modules log U0402. One root fault, two codes.

First Things to Check (In This Order)

Manual Park Release Lever

Under the hood there is a small lever used to manually release the park lock when the car needs to be moved without power — for loading onto a flatbed transporter, for example. If this lever is accidentally in the raised or half-actuated position, it interferes with the park lock confirmation circuit and will trigger P1707-77 immediately after startup.

This is the free check. Press the lever firmly to the down position until it seats fully, clear the fault codes, and do a cold ignition cycle. Five minutes. Worth ruling out before anything else.

Battery Voltage and Charging System

Low battery voltage is a documented trigger for U0402 on Land Rovers. When resting voltage drops below around 12.4V, CAN bus communication between modules becomes unreliable. The TCM outputs noisy data, other modules flag it, and the whole system can refuse to cooperate. Check resting voltage with a multimeter, then load-test the battery properly — a surface reading above 12V does not mean the battery is healthy under load. A weak battery on a Discovery 5 can produce a cascade of apparently unrelated fault codes across multiple systems.

Park Lock Solenoid Circuit

If the lever is fine and the battery tests healthy, the park lock solenoid or its wiring is next. The solenoid sits in the valve block. Failure modes covered by P1707-77 include short to ground, short to power, open circuit, and high resistance. Check the harness connector at the gearbox bell housing for corrosion or pushed-back terminals, then measure solenoid resistance against the specification in the workshop manual. A Land Rover specialist with the correct wiring diagram can run this in under an hour.

What the Sub-Codes Actually Mean

The -77 qualifier in P1707-77 is a JLR-specific sub-code meaning the fault type is a circuit short to ground or open circuit. Other P1707 variants — -11, -72, and others — indicate different electrical failure modes. The -77 narrows the search to the solenoid circuit wiring rather than a mechanical failure in the park lock pawl assembly itself.

The (00) in U0402(00) is simply the base code with no additional sub-classification. On Discovery 5s this almost always traces back to the TCM broadcasting bad park lock status rather than the TCM hardware itself having failed — which matters, because replacing a TCM is expensive and is rarely the actual fix.

Repair Paths, From Cheapest to Most Involved

  • Manual park release lever reset — free, try this first
  • Battery replacement or conditioning charge — inexpensive
  • Wiring harness inspection and connector repair — manageable at an independent transmission specialist
  • Park lock solenoid replacement — requires valve block access, not a driveway job
  • Valve block replacement — if the solenoid tests bad or internal valve body wear is found. After fitting a new valve block, adaptation routines must be completed using JLR Pathfinder or an equivalent dealer-level diagnostic tool. Skipping this step causes the gearbox to shift incorrectly even after a correct physical repair
  • TCM replacement and reprogramming — least common root cause, but documented in cases where all electrical checks pass and the fault returns after clearing

Don’t Try to Drive It

The gearbox is in fault mode. It refuses gear engagement to protect itself, and attempting to force it risks damaging the park pawl or the selector assembly. Have the vehicle transported to a specialist with JLR Pathfinder access. Generic OBD readers can pull the fault codes, but clearing them and running post-repair adaptation routines requires the dealer-level diagnostic system — this is not optional.

Cold Start Edge Case

P1707-77 can appear at very low temperatures without any component failure. The ZF 8HP relies on oil pressure to release the park lock, and in extreme cold the fluid thickens enough that the pump struggles to build sufficient pressure before the TCM times out on the confirmation signal. If the fault appeared on a very cold morning and the vehicle has been otherwise trouble-free, a transmission fluid change to a cold-weather-rated grade may resolve it without further repair.

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