Bosch Boiler Error Codes: US Greenstar EA, C6 and E9

By Ray Bennett · Updated 17 October 2026 · 7 min read · Checked against the maker’s manual

Bosch Boiler Error Codes: US Greenstar EA, C6 and E9

Short answer

Bosch's US Greenstar chart calls EA a missing flame, C6 a fan that is not running, and E9 a heat exchanger or flue gas safety limit trip. These codes guide the service checks; they do not identify a replacement part by themselves. Use the Greenstar model number and note whether Reset flashes.

Bosch boiler error codes EA, C6 and E9 lead to different checks on the US Greenstar. Bosch’s service chart separates ignition, the combustion fan and safety temperature limits, even when the symptom you notice is simply no heat.

This page covers Greenstar ZBR21/28/35/42-3A and ZWB28/35/42-3A in Bosch’s US/CA manuals. Bosch identifies ZBR as the heating boiler family, which can serve an indirect hot water tank, and ZWB as the combi family for heating and domestic hot water.

Before you start

What to carry: a torch and your phone. Photograph the complete code, the accessible model label and whether the Reset button flashes. Include whether the problem affects the rooms, the hot taps, or both.

Gas or electric? These Greenstar manuals cover gas condensing boilers. If the fault is on a Bosch outdoor heat pump, use Bosch IDS heat pump codes instead.

Is it a Buderus? The GB142 and SSB controls already have their own Buderus boiler error code guide. Do not translate a code from that page into a Greenstar code. This guide uses US Greenstar documents, not Worcester manuals.

Renting? Send the fault details to the person responsible for heating repairs. Keep access to the boiler clear for the service visit.

Deal with these conditions before a reset. Overheating or gas that will not shut off: Bosch’s Greenstar owner manual says to keep the pump’s electrical supply on and shut off gas outside the appliance. Treat that as urgent service, not a reason to cycle the power.

Gas smell: Bosch says not to light an appliance, touch switches or use a phone in the building. Warn other occupants without ringing a bell, then leave and immediately call the gas supplier from outside. Follow their instructions; if they cannot be reached, call the fire department.

A carbon monoxide alarm or suspected CO leak: Bosch’s owner instructions say to warn your neighbors, leave the building immediately and call an approved contractor. Have any defects corrected. Do not stay beside the display to diagnose it.

Smell flue gas? Bosch says to switch the appliance off, open windows and doors, and inform a trained and certified heating contractor. The overheating exception above still applies.

Has any part been under water? Bosch says not to use the boiler. Call a trained and certified service technician immediately to inspect it and replace any submerged controls.

F7 or FA: Bosch’s Greenstar chart reports flame when the burner is off, or after gas has been switched off. Leave the investigation and restart to a heating technician. Its checks include electrodes and, for FA, the gas valve.

Bosch reserves installation and service for a trained and certified contractor. Keep the cabinet closed and leave gas pressure, combustion adjustments and electrical testing to that person.

What the US Greenstar display is reporting

Three US Greenstar faults, three starting points. Bosch's table 38 gives further tests for each; the code alone does not choose the replacement part.

These selected entries come from Bosch’s US Greenstar service manual 6721853955, table 38. Sensor and control checks belong to the service contractor.

Display	Meaning in this Greenstar chart
EA	Flame not detected; safety time expired
C6	Fan not running
E9	Heat exchanger high limit or flue gas temperature limiter tripped
E2 / E3	Supply / additional supply temperature sensor fault
A7	Domestic hot water temperature sensor fault
A8	Communication fault
Ad	Previously recognized hot water tank sensor no longer detected
CC	Outdoor temperature sensor not recognized
d3	Temperature high limit fault, external guard trip or limiter lockout
F0	Internal fault
F7 / FA	Unexpected flame indication with burner off / after gas is switched off

EA: the boiler did not detect a flame

Check 1Ignition and fuel supply tests before any parts decision

Give the technician the ignition history

Bosch’s EA checks include the gas shutoff, inlet gas pressure, electrical supply, grounding and electrodes. Its list also checks the flue, gas and air mixture, condensate trap and heat exchanger cleanliness.

Tell the contractor whether the fault followed a gas interruption, a service visit or other work near the boiler. These details help them choose where to begin within Bosch’s list. You do not need to open the boiler or operate the gas valve to collect them.

Air supply matters for a room air installation. Bosch’s owner manual says never to cover or reduce ventilation openings in doors, walls or windows, and never to operate with insufficient combustion air. Leave an obstructed air path for correction before restarting.

EA is not a verdict on the igniter. If you also see water escaping, the separate boiler leaking water guide helps describe where it starts; trap cleaning remains service work.

C6 and E9 need different service checks

Check 2Electrical and fan diagnosis; parts only if testing finds a defect

C6: have the fan and its leads checked

Bosch’s C6 entry says the fan is not running and directs the contractor to check the fan leads and the fan. That makes the connection part of the diagnosis, rather than an automatic instruction to replace the motor.

The boiler not working guide covers the wider no heat complaint, while this fault needs the Greenstar fan checks.

Check 3Safety limit, pressure and circulation investigation

E9: report the gauge reading without refilling

Bosch’s E9 entry identifies either the heat exchanger safety high limit or the flue gas temperature limiter. Its service list checks those limits and their leads, operating pressure, the pump capacitor, the control fuse, trapped air and the domestic hot water side heat exchanger.

If the gauge is visible, send its reading with the fault. Leave filling, bleeding and pump tests to the contractor rather than assuming E9 means low water. The boiler pressure guide explains the water side, and the circulator pump guide explains circulation symptoms.

d3 is a separate entry. Bosch includes an external guard trip and a temperature limiter lockout under d3. Tell the contractor about any separate safety control fitted beside the boiler; do not bridge its connections to clear the message.

How Bosch’s owner reset works

Bosch's US Greenstar owner instructions choose the reset method by whether the Reset button flashes. Deal with the safety conditions above first.

Check 4An owner control step, followed by service if the fault persists

Use the Reset button's state

Once the safety warnings above have been ruled out, follow Bosch’s Greenstar reset instructions below.

If Reset is flashing: Bosch says to hold that button until the reset confirmation appears. Its page 28 illustration shows two horizontal bars on the display. The boiler then starts again and shows the current supply temperature.

If Reset is not flashing: Bosch says to switch the appliance off and on again. This is the ordinary fault procedure, subject to the overheating exception in the safety box.

If the fault persists: Bosch directs the owner to a trained and certified heating contractor with the code, appliance type and serial number. Leave it at that rather than repeating resets while the house stays cold.

What each one takes

Fault family	What the repair involves
EA	Ignition, gas, air and condensate diagnosis
C6	Checking the fan and its electrical leads
E9 or d3	Identifying the safety limit involved and testing its cause
Sensor or communication fault	Wiring and component checks before replacement
F7 or FA	Investigation of an unexpected flame indication

Bosch’s owner manual warns that a system disabled by a fault can freeze in cold weather. If heat cannot be restored, ask the contractor to arrange suitable freeze protection while the repair is pending.

Questions people ask

Where can I find the Bosch Greenstar error code PDF?

Use Bosch’s US boiler manual library. This page uses wall boiler service manual 6721853955, table 38 on pages 74 and 75, and operating manual 6720818061. Match the model and document edition.

What if my Bosch boiler shows 6A?

Keep that code as written and check the full model. For a GB142, use the Buderus guide linked near the top. Bosch’s Greenstar table lists EA for no flame detected.

Does this cover a Greenstar combi boiler?

It covers the ZWB28/35/42-3A models listed above. Bosch identifies those as combi boilers in the same US manual. Do not extend the chart to a different Bosch combi control.

Does E9 mean the pump has failed?

Bosch includes pump testing among its E9 checks, alongside safety limits, pressure and other items. It takes those tests to identify the repair.

Why did the warning sound stop while the code stayed?

Bosch says a button press mutes the warning tone. Read the code and Reset button state separately; a quiet boiler is not confirmation that the fault has cleared.

The other boiler guides: Buderus codes, no heat, pressure problems and circulator problems.

Sources: Bosch Greenstar US/CA Installation and Service Instructions 6721853955 (2025/11), table 38, pages 74 and 75; Greenstar US/CA Operating Instructions 6720818061 (2024/05), safety instructions and troubleshooting page 28.