Webasto Thermo Pro 90 Series

Questions and Answers

A
Alexander WilsonAug 17, 2025
Why Webasto Water Heater does not respond and the operation indicator remains off?
- S
  Scott Turner MDAug 17, 2025
  If your Webasto Water Heater doesn't respond and the operation indicator is off, the issue may be with the operating voltage. Check fuses F2 and F3, and measure the supply voltage at heater connector X8, Pin 12. Alternatively, the operation indicator itself (control element or ON/OFF switch) could be the problem. Measure the supply voltage at connector X9, Pin 11 and check continuity at connector X9, Pin 12 to ground. Another possibility is the switch. In this case measure the supply voltage at switch S4, Pin A and check continuity at switch S4, Pin F to ground.
B
Brendan FloresAug 19, 2025
What to do if Webasto Thermo Pro 90 will not start, briefly starts up, switches to afterrunning, and operating indicator flashes?
- J
  Jacob BurkeAug 19, 2025
  If your Webasto Water Heater won't start, briefly starts then switches to afterrunning, with the operating indicator flashing, it could be due to several reasons. It could be the DP42 fuel pump. Check the fuel line connection at the fuel pump DP42 and ensure clear passage and seating of the flat spring contacts of the fuel pump line. Measure the cold resistance of fuel pump DP42 at 25 °, check setpoints. It may also be a combustion air fan, U4840 coolant pump, coolant temperature sensor or glow plug. In these cases connect Webasto Thermo Test and evaluate. Also check coolant level.
J
Jennifer GonzalezAug 22, 2025
Why is there excessive smoke from Webasto Thermo Pro 90 during start-up phase?
- T
  todd01Aug 22, 2025
  Excessive smoke during the start-up of your Webasto Water Heater can be caused by several factors. It could be due to the glow plug, DP42 fuel pump or combustion air fan. In these cases connect Webasto Thermo Test and evaluate. Check the fuel level and fuel take-off from the fuel tank. Also, make sure that the exhaust and air intake systems are not clogged. A clogged fuel pipe in the burner unit can also be the reason. In this case removal and visual inspection is needed. Replace the burner unit if the fuel pipe is clogged.
D
Diane ReynoldsAug 25, 2025
What to do if Webasto Water Heater goes out prematurely?
- B
  Brianna CoxAug 26, 2025
  If your Webasto Water Heater goes out prematurely, it might be due to undervoltage detection. Check fuses F2 and F3, and use Webasto Thermo Test PC Diagnostics to check the supply voltage. Other potential causes include the DP42 fuel pump, a stiff combustion air fan, fuel integration issues, problems with the coolant circuit, or blockages in the exhaust and air intake systems. It may also be due to the burner unit. For DP42 fuel pump, combustion air fan or burner unit refer to symptom B or C respectively. Check integration in vehicle's coolant circuit. For fuel integration or exhaust system and air intake system refer to symptom C.
E
Emily JohnsonAug 27, 2025
Why is there excessive smoke from Webasto Thermo Pro 90 during heating phase?
- R
  Ronald UnderwoodAug 27, 2025
  Excessive smoke during the heating phase of your Webasto Water Heater can be related to the DP42 fuel pump, fuel integration, exhaust and air intake systems, or the burner unit. For DP42 fuel pump, fuel integration, exhaust and air intake systems or burner unit refer to symptom B or C respectively.
D
Donald GarrettAug 30, 2025
Why Webasto Thermo Pro 90 Water Heater has irregular combustion noise?
- B
  Benjamin Webb MDAug 30, 2025
  Irregular combustion noise in your Webasto Water Heater can be caused by the DP42 fuel pump or the burner unit. For DP42 fuel pump or burner unit refer to symptom B or C respectively.
G
Gordon AndrewsSep 2, 2025
Why Webasto Thermo Pro 90 is running, but vehicle interior is cold?
- C
  Christine WagnerSep 2, 2025
  If your Webasto Water Heater is running but the vehicle interior remains cold, check fuse F1 and the coolant temperature. The issue might be with the vehicle blower.
H
Harry LewisSep 5, 2025
Why is there fuel odour from Webasto Water Heater?
- C
  Catherine BauerSep 5, 2025
  A fuel odor from your Webasto Water Heater could stem from fuel integration issues, the DP42 fuel pump, or the glow plug. For fuel integration, DP42 fuel pump, or glow plug refer to symptom C, B or A respectively.
E
Elizabeth LewisSep 8, 2025
Why is there excessive smoke from Webasto Water Heater during afterrunning phase?
- D
  David BryanSep 8, 2025
  Excessive smoke during the afterrunning phase of your Webasto Water Heater can be caused by the exhaust system and air intake system. For exhaust system and air intake system refer to symptom C.
N
nicholaslaraSep 10, 2025
Why is there smell of exhaust in vehicle interior from Webasto Water Heater?
- E
  Erin SchneiderSep 12, 2025
  If you smell exhaust inside the vehicle from your Webasto Water Heater, it's likely due to issues with the exhaust system and air intake system. For exhaust system and air intake system refer to symptom C.

Summary

Introduction to Webasto Thermo Pro 90

Contents and Purpose of the Manual

Overview of the manual's content and intended audience for repairs.

Understanding Signal Words

Explains the meaning of CAUTION, ATTENTION, and NOTE for safe operation and maintenance.

Additional Documentation Required

Lists essential documents required for installation and operation alongside the manual.

Legal Requirements and Safety Information

Covers statutory regulations, general safety information, and installation compliance for vehicle heaters.

Spare Parts Information

Provides information on how to identify and obtain genuine Webasto spare parts for the heater.

General Description of the Heater

Combustion Air Fan Function

Details the function of the combustion air fan, supplying air for the combustion process.

Overheating Protection and Temperature Sensors

Describes components that monitor coolant temperature and prevent overheating.

Glow Plug and Burner Unit

Explains the role of the glow plug and burner unit in ignition and combustion.

Combustion Chamber Function

Outlines the function of the combustion chamber in assisting fuel/air mixture combustion.

Exhaust Gas Temperature Sensor

Explains how the exhaust gas temperature sensor detects flame and monitors exhaust temperatures.

U4840 Coolant Pump Operation

Describes the U4840 coolant pump's function in circulating coolant during heater operation.

Control Unit Functionality

Details the control unit's role in managing heater functions, sequences, and monitoring.

DP42 Fuel Pump

Explains the DP42 fuel pump's function in supplying fuel to the heater.

Functional Description of the Heater

Heater Start-up and Residual Heat

Covers the heater's start-up sequence and residual heat utilization based on coolant temperature.

Start and Control Mode Operation

Describes automatic heating, output modulation, and vehicle fan integration.

Heater Switching Off Procedure

Details the process when the heater is switched off, including afterrunning phases.

Heater Functions in ADR Vehicles

Explains operational requirements and safety functions for heaters in ADR-certified vehicles.

Malfunctions and Troubleshooting Guide

General Information on Faults

Provides an overview of identifying and rectifying faults in the Thermo Pro 90 heater.

Procedure for Faults and Lock-outs

Outlines the steps to follow when the heater experiences faults, malfunctions, or lock-outs.

Causes of Malfunctions

Lists potential causes for heater malfunctions, including component failures and operating conditions.

Deleting Fault Codes

Explains the procedures for clearing fault codes from the heater's memory.

Heater Lock-out Procedure

Details how to unlock the heater after it has entered a lock-out mode due to faults.

Fault Code Table and Flash Codes

A comprehensive table mapping fault codes to error messages, causes, and actions.

Troubleshooting by Fault Symptoms

A guide to diagnosing and resolving issues based on observed symptoms and their possible causes.

Function Checks and Settings

General Information on Checks

Introduces checks and settings for the heater and its components in various states.

Vehicle Function Checks

Describes how to perform operational checks on the heater when installed in a vehicle.

Setting the CO2 Content

Explains checking and setting CO2 content for optimal combustion performance.

Testing Individual Components

Details specific tests for individual heater components like sensors, pumps, and fans.

Servicing the Webasto Heater

Working on the Heater Unit

General guidelines and safety precautions for performing work on the heater itself.

Working on the Heater in the Vehicle

Safety notes regarding working on the heater while it is installed in the vehicle.

Heater Test Run Procedure

Instructions for conducting a test run after servicing or installation to ensure proper operation.

Regular Servicing Checks

Lists essential servicing jobs to maintain heater reliability before and after each heating period.

Heater Removal and Installation

Step-by-step procedures for safely removing and reinstalling the heater into the vehicle.

Visual Inspection of the Burner Unit

Guidelines for visually inspecting the burner unit for damage or wear that may require replacement.

Restarting the Heater

Procedures for restarting the heater after installation, including bleeding coolant and fuel systems.

Repairing and Replacing Heater Components

General Repair Information

Introduces repair jobs performed on the heater after removal, emphasizing warranty and spare parts.

Heater Component Layout

Illustrates the arrangement and identification of major components within the heater assembly.

Work on Stripped Down Components

Procedures for cleaning and visually inspecting heater components after disassembly.

Electrical Connections

Guidance on handling electrical connections, including unplugging and reestablishing them.

Control Unit Removal and Installation

Step-by-step instructions for removing and installing the heater's control unit, with precautions.

U4840 Coolant Pump Replacement

Detailed procedure for removing and installing the U4840 coolant pump, including lubrication and torques.

Overheating Protection Replacement

Instructions for removing and installing the overheating protection sensor, with critical installation notes.

Coolant Temperature Sensor Replacement

Procedure for removing and installing the coolant temperature sensor, including lubrication and tightening.

Combustion Air Fan Replacement

Steps for removing and installing the combustion air fan, including handling the control unit.

Burner Unit and Glow Plug Replacement

Instructions for removing and installing the burner unit and glow plug assembly, including securing them.

Burner Head Replacement

Procedure for removing and installing the burner head, including alignment and securing the V-clip.

Heat Exchanger Replacement

Steps for removing and installing the heat exchanger, including cleaning soot deposits.

Exhaust Gas Temperature Sensor Replacement

Detailed instructions for removing and installing the exhaust gas temperature sensor, with handling precautions.

Thermo Pro 90 HDD Specifics

General Information on HDD Model

Provides an overview of differences between the Thermo Pro 90 HDD and the standard model.

Technical Data for HDD Model

Presents specific technical specifications for the Thermo Pro 90 HDD model.

Coolant Pump with EMC Suppressor Module

Details the replacement procedure for the EMC suppressor module on the U4840 coolant pump.

HDD Combustion Air Fan

Notes the special design of the combustion air fan for the Thermo Pro 90 HDD model.

Fuel Requirements for HDD

Specifies the required diesel fuel type and compliance standards for the Thermo Pro 90 HDD.

HDD Control Unit

Instructions related to the control unit for the Thermo Pro 90 HDD, including installation.

HDD Electrical Components

Mentions electrical component considerations, including minimum bending radius for wiring harnesses.

HDD Wiring Diagrams Reference

Refers to the section detailing wiring diagrams specific to the Thermo Pro 90 HDD model.

Packaging, Storage, and Shipping Guidelines

General Packaging and Shipping Information

Provides guidelines for cleaning, packing, and protecting heaters for transport and storage.

Storage and Transportation Conditions

Outlines conditions and restrictions for storing and transporting the heater.

Webasto Wiring Diagrams

Wiring Diagrams for Thermo Pro 90

Lists available system wiring diagrams for the standard Thermo Pro 90 model and its control elements.

Wiring Diagrams for Thermo Pro 90 HDD

Lists available system wiring diagrams for the Thermo Pro 90 HDD model and its control elements.

Thermo Pro 90 MultiControl with Battery Disconnector Wiring

Provides the wiring diagram for the Thermo Pro 90 with a MultiControl element and battery disconnector.

Thermo Pro 90 SmartControl (ADR) Wiring

Shows the wiring diagram for the Thermo Pro 90 with a SmartControl element for ADR applications.

Thermo Pro 90 On;Off Switch (ADR) Wiring

Presents the wiring diagram for the Thermo Pro 90 with an on/off switch in ADR configuration.

Thermo Pro 90 ThermoCall TC4 Wiring

Details the wiring diagram for the Thermo Pro 90 when used with a ThermoCall TC4 unit.

Thermo Pro 90 UniBox Wiring

Shows the wiring diagram for the Thermo Pro 90 when integrated with a UniBox.

Thermo Pro 90 MultiControl (Analogue) Wiring

Provides the wiring diagram for the Thermo Pro 90 with a MultiControl element using an analogue connection.

Thermo Pro 90 HDD MultiControl Wiring

Shows the wiring diagram for the Thermo Pro 90 HDD with a MultiControl control element.

Thermo Pro 90 HDD SmartControl (ADR) Wiring

Details the wiring diagram for the Thermo Pro 90 HDD with a SmartControl element for ADR.

Thermo Pro 90 HDD On;Off Switch (ADR) Wiring

Presents the wiring diagram for the Thermo Pro 90 HDD with an on/off switch in ADR configuration.

Cable Cross-Sections

Lists cable cross-sections and their corresponding lengths for wiring installations.

Cable Colour Legend

Provides a legend for cable colours used in wiring diagrams, with abbreviations and their meanings.

Heater Connections Overview

Illustrates the main connection points on the heater unit for various components.

Control Unit Plug Assignments

Shows the physical layout and assignments of plugs on the heater's control unit.

Legend to Wiring Diagrams

Explains the meaning of different items, descriptions, and remarks found in the wiring diagrams.

Legend for Wiring Diagram Comments

Provides explanations for various numbered comments found within the wiring diagrams.

Legend for Plug X8, 12-pin

Details the pin assignments and remarks for the 12-pin plug connector X8.