Thetford C402 Cassette Toilet
(C402-C / C402-X)
Official technical documentation and troubleshooting for the Thetford C402 cassette toilet, fitted to several UK motorhomes including Auto-Trail's Expedition range. The C402-C and C402-X have their own integrated flush-water tank; a third variant in the same manual, the C403-L, instead draws flush water from the vehicle's own water system - not covered by this entry unless confirmed fitted elsewhere.
View official manual on manufacturer's site ↗Technical Specifications
Master Troubleshooting & Diagnostics Matrix
| Error Code / Indicator | Symptom / Meaning | Possible Cause | Step-by-Step DIY Fix | Suggested Actions |
|---|---|---|---|---|
N/A | The waste-holding tank cannot be removed | The blade is partly or fully open. | Fully close the blade (turn the control knob clockwise until it clicks), then try removing the waste-holding tank again. | DIY Safe |
N/A | The blade does not move | The blade mechanism is blocked. | Loosen the cap on the pour-out spout to release the blockage, then try operating the blade again. | DIY Safe |
N/A | The waste-holding tank leaks | The seal has dried out, or has worn from use. | If the seal has just dried out, spray it with Thetford Seal Lubricant. If it's worn, the seal itself needs replacing - never use Vaseline or vegetable oil as a substitute lubricant, as these can cause further leaks. | DIY Safe |
Frequently Asked Questions
Q: How do I prepare the toilet before using it for the first time?
Remove the waste-holding tank via the service door, stand it upright with the pour-out spout facing up, remove the spout cap, add the correct dose of Thetford waste-holding tank additive plus around 3 litres of water so the bottom of the tank is covered, then replace the cap and slide the tank back in. On the C402-C and C402-X, also fill the integrated flush-water tank via the waterfill funnel with the correct dose of Thetford flush-water additive plus clean water.
Q: How do I use and flush the toilet?
Open the blade by turning the control knob anticlockwise; it can be left open or closed while the toilet is in use, but should always be closed fully afterwards (turn clockwise until it clicks). To flush, make sure the blade is open and hold the flush button down for several seconds. Don't leave water standing in the bowl when the toilet isn't in use, and don't travel with water in the bowl - both risk water damage to the vehicle.
Q: How do I empty the waste-holding tank?
Pull the tank's handle up until fully extended and remove it via the service door, take it to an authorised waste disposal point, stand it upright with the pour-out spout up, remove the cap, then hold the vent button in while emptying so it doesn't splash. Add around 5 litres of water, replace the cap, shake gently to clean the inside, then empty it again before sliding it back into the toilet.
It needs emptying once the tank's red level indicator lights up - at that point it can only be used for two more flushes, and shouldn't be travelled with when more than three-quarters full.
Q: How do I empty the flush-water tank (C402-C and C402-X only)?
Place a bowl under the drain tube, pull the tube out to let the remaining water drain into it, then close the tube and put it back. Only do this if the toilet won't be used for a while - Thetford recommends travelling with the flush-water tank empty, or at most half full, to avoid water damage.
Q: Can I use the toilet in cold weather, and how do I put it into storage?
It's fine to use in cold weather as long as it's kept in a heated part of the vehicle - otherwise there's a risk of the water freezing. If it won't be heated, or before long-term storage, fully drain the toilet and the vehicle's water system: empty the flush-water tank (where fitted) and the waste-holding tank, clean and dry the whole toilet, then open the blade and loosen the pour-out spout cap to ventilate the waste-holding tank.