The Thermo Top C stores its faults in memory and reports them over the W-Bus (command 0x56 01). The codes are the same ones VCDS reads on module 18 (auxiliary heater), the WTTC app and web page, or any W-Bus reader. Each code is in hexadecimal; many come in two versions: 0x0_ for a short circuit and 0x8_ for an open circuit (broken wire or loose connector).
| Code | Meaning | What to check |
|---|---|---|
| 0x01 | Defective control unit | Pull and refit its fuse; if it comes back, the control unit needs a workshop. |
| 0x02 | No start | Fuel: tank level, line and filter, air in the line; metering pump and its wiring; air intake and exhaust not blocked; glow plug. |
| 0x82 | No start during test run | Fuel: tank level, line and filter, air in the line; metering pump and its wiring; air intake and exhaust not blocked; glow plug. |
| 0x03 | Flame failure (goes out during start-up) | Fuel: tank level, line and filter, air in the line; metering pump and its wiring; air intake and exhaust not blocked; glow plug. |
| 0x83 | Flame failure (while running) | Fuel: tank level, line and filter, air in the line; metering pump and its wiring; air intake and exhaust not blocked; glow plug. |
| 0x04 | Supply voltage too high | Alternator regulator and voltage with the engine running (should not exceed ~15 V). |
| 0x84 | Operating voltage too low | Battery charge, terminals, grounds and voltage drop in the wiring (the glow plug draws a lot of current at start-up). |
| 0x05 | Flame detected before combustion | Flame sensor / glow plug; leftover diesel in the chamber after failed starts. |
| 0x85 | Flame detected after combustion | Flame sensor / glow plug, and that the metering pump does not drip when stopping. |
| 0x06 | Overheating | Coolant level, air in the circuit, water pump, kinked hoses or thermostat. |
| 0x07 | Locked out (interlock) | It locks out after several failed starts or overheating: find the cause (other codes) and unlock it (see below). |
| 0x87 | Permanent lock-out | As for 0x07, but a diagnostic tool is needed to unlock it. |
| 0x08 | Metering pump: short circuit | Wiring and connector of the metering pump (usually near the tank). |
| 0x88 | Metering pump: failure / open circuit | Metering pump connector, broken wire or faulty pump. |
| 0x09 | Combustion air fan: short circuit | Fan motor and its wiring. |
| 0x89 | Combustion air fan: open circuit | Fan connector and wiring. |
| 0x0A | Glow plug / flame monitor: short circuit | Glow plug and its wiring. |
| 0x8A | Glow plug / flame monitor: open circuit | Burnt-out glow plug or loose connector. |
| 0x0B | Circulation pump: short circuit | Water pump and its wiring. |
| 0x8B | Circulation pump: open circuit | Water pump connector or faulty pump. |
| 0x12 | W-Bus communication failure | The W-Bus wire (the “black” one on the controller connector), common ground and, with WTTC, the TJA1020 module. |
| 0x92 | Command refresh failure | The controller stopped sending the keep-alive while it was running: W-Bus wire or the controller/timer itself. |
| 0x14 | Temperature sensor: short circuit | The heater's temperature sensor and its wiring. |
| 0x94 | Temperature sensor: open circuit | Temperature sensor connector or broken wire. |
| 0x15 | Combustion air fan blocked | Something stopping the fan from turning (dirt, rust, soot). |
| 0x97 | Overheat sensor position wrong | That the sensor is correctly seated in its housing. |
| 0x98 | Power supply interruption | Fuse, terminals and the heater's +12 V and ground wiring. |
| 0x9C | Intelligent undervoltage detection | As for 0x84: battery and connections. |
After several failed starts in a row or overheating, the Thermo Top C locks itself out and stops obeying until it is unlocked. This is done by clearing the fault memory with a diagnostic tool (VCDS, module 18) or with the procedure given in your installation's manual. Find the cause first: otherwise it will lock out again.
Code names according to the error table of libwbus. Webasto and Thermo Top are trademarks of their owners; WTTC is not related to them.