Honeywell 7800 Series Fault Codes: Causes and Corrective Actions

2. Resetting a Lockout

To reset a 7800 Series lockout, press and release the reset button on the relay module, or use the remote reset if one is wired. The control returns to standby and runs a normal startup sequence on the next call for heat. A lockout indicates the control detected an unsafe condition. Resetting clears the code, not the condition. Repeated resets after a flame failure put unburned fuel into the combustion chamber, which creates an explosion hazard. Reset once, observe the startup sequence, and record where in the sequence the failure occurs and what code results. The control’s fault history stores recent lockouts with cycle counts, so prior faults can be reviewed even after a reset.

If the same code returns, diagnose before resetting again. Never jumper an interlock, switch, or flame signal to keep a burner running. Flame safeguard service should be performed by a qualified burner technician.

3. The Complete 7800 Series Fault Code Table

The display shows LOCKOUT followed by the code number and fault message.

Don’t see your fault code, or seeing a code that returns after every reset? Contact TCI Supply or call 1-800-755-2883. If the diagnosis points to hardware, we stock the replacement.

4. The Lockouts You’ll Actually See

Four fault families account for most 7800 Series service calls.

Pilot and main flame failures (codes 16, 28, 17, 19)

Codes 16 and 28 mean no flame was proven during the pilot period. Codes 17 and 19 mean flame was lost after it had been established.

1. Read the flame signal on the KDM during a supervised trial. A signal near or below 1.25 Vdc identifies a detection problem before anything else is opened.
2. Check the scanner or flame rod. Clean the UV scanner lens, check for moisture in the scanner conduit, and verify the sighting is aimed at the flame envelope. A scanner that reads the pilot at low fire but loses the main flame at high fire is a sighting problem.
3. Check fuel: pilot gas pressure, main fuel pressure, and valve operation. Run the pilot turndown test after any pilot adjustment.
4. Check the ignition transformer and electrode gap.
5. Verify the flame amplifier matches the detector type and the amplifier is seated. Replace the amplifier, then the detector, if the signal cannot be brought into specification.

False flame (codes 7, 9, 15, 18, 51)

The control sensed flame when none should be present. This lockout protects against fuel entering a chamber the control believes is already burning.

1. Confirm no flame or glowing refractory is present in the chamber. Hot refractory can hold a UV scanner in.
2. Check for ignition noise coupling. F and G leads must run in their own conduit, away from ignition and line voltage wiring.
3. Reseat the flame amplifier. A failed amplifier that reports flame continuously produces these codes at every cycle.
4. Code 51 means the signal is too strong to be valid, which usually indicates a shorted detector lead or a failed amplifier rather than an actual flame.

Airflow and running interlock (codes 13, 21, 23, 29, 30, 32)

The running interlock string, which includes the airflow switch, opened or failed to close at the required point in the sequence.

1. Verify the combustion fan is running and the air intake and proving line are clear. A plugged sensing line is the most common airflow switch failure.
2. Verify the interlock string closes: measure for line voltage at terminal 7 during PREPURGE. Voltage present means the string is closed; absent means a switch in the string is open.
3. If an Expanded Annunciator is installed, the letter after the code identifies the first switch that opened. Diagnose that switch, not the string.
4. Codes 11, 12, and 13 are the reverse case: the string was closed when it should have been open, which means a welded, jumpered, or stuck switch. Find it and replace it. Do not leave a jumpered interlock in service.

AC power quality (codes 2, 3, 4, 5)

The control monitors its own supply and locks out on dropout, low voltage, frequency error, or line noise.

1. Measure line voltage at the module under load. Low line at the boiler during other equipment starts is a common cause of code 5.
2. Check neutral and earth ground connections. Code 109 in the internal fault range points the same direction.
3. Recurring codes 2 through 5 on a stable line usually indicate a shared circuit problem or a failing transformer upstream, not a module failure.

5. When the Code Means Hardware

The 7800 Series is a modular system. The field wiring lands on a Q7800 subbase, and every active component plugs in: the relay module, the flame amplifier, the purge card, and the KDM. A hardware diagnosis is a component swap, not a rewiring job.

Relay module. Codes 24, 25, 35 through 40, and the internal fault range 94 through 127 are self-test and output check failures inside the module. Honeywell’s corrective action for each is the same: reset once, and replace the relay module if the fault returns. Code 110 has a hard rule: configuration jumpers changed after 200 hours of operation cause a permanent lockout, and the module must be replaced. The replacement module must match the model and be paired with the correct purge card and amplifier; the subbase and field wiring stay in place.

Flame amplifier. Codes 7, 8, 46, and 51 point at the amplifier first. It plugs into the side of the relay module and swaps in seconds. The amplifier must match the detector type: UV, UV self-check, infrared, or flame rod.

Flame detector. When the amplifier checks out and the flame signal still cannot reach specification, the scanner or flame rod is next. UV tubes lose sensitivity with age and fail gradually, which shows up as marginal flame signal before it becomes a lockout.

Purge card. Codes 1 and 6. The card sets purge timing and is a plug-in replacement. Replace with the same timing designation.

KDM. The display is not part of the safety chain. A scrambled or dark display is a KDM or connection problem, not a lockout; remove and reinstall the KDM and reset. The burner runs without it.

Part numbers for each component are below.

6. 7800 Series Replacement Parts Quick Reference

Common relay modules, amplifiers, scanners, and accessories for fault repairs. Every part below is something we stock or source.

Amplifier and detector must match: UV scanners pair with R7849 amplifiers, flame rods with R7847, infrared with R7852. Verify the pairing before ordering either half.

Replacing an older Honeywell primary rather than repairing a 7800? Our flame safeguard replacement guide covers the R4795, BC7000, and RA890 upgrade paths, and our RM7800 vs RM7823 vs RM7890 comparison covers choosing between current models. Or send us the model number from your control and we’ll quote the exact replacement.

7. Get the Burner Back Up: Source Your Replacement From TCI Supply

If the diagnosis landed on hardware, send us the model number from your relay module, amplifier, or scanner and the fault code you’re seeing. We’ll confirm the exact replacement and get it moving. We stock the common 7800 Series relay modules, amplifiers, and Purple Peeper scanners, and we source discontinued flame safeguard parts through the secondary market. Flame safeguard replacement should be installed and commissioned by a qualified burner technician. Contact TCI Supply or call us at 1-800-755-2883.

Frequently Asked Questions