Conquest 90 Gas Furnace Troubleshooting Manual

Getting Your Bryant Conquest 90 Running Again

The Conquest 90 is one of those mid-efficiency furnaces Bryant made in large numbers between roughly 2008 and 2016. It uses a direct-spark ignition system, a hot surface ignitor, and a dual-stage gas valve. The control board is where most problems show up, and the board talks to you through LED blink codes. Before you start swapping parts, you need to watch the board during a call for heat. I found the original Bryant troubleshooting guide for this unit to be adequate but frustratingly sparse on real-world edge cases. They list the LED codes, which is helpful, but they don't always tell you what actually fails in the field. The document itself is typically distributed as a PDF under Bryant part number 391137 or similar. Search for "Bryant Conquest 90 troubleshooting manual PDF" and you'll find several HVAC supplier sites hosting it. Some are scanned copies of the paper manual, others are the newer revised versions. The revised versions added notes about certain ignition transformer failures that the first print run omitted. Here is how I actually use the manual when a unit is on the floor. You pull the control board cover and watch the LEDs while the furnace goes through its startup sequence. The board has two LEDs. One shows the power status, the other shows fault codes. During normal operation you won't see much. When something goes wrong, the fault LED blinks a pattern. Four quick blinks means the ignitor failed to light within the trial period. Five blinks means the flame sensor isn't picking up a signal after ignition. Six blinks is a pressure switch open fault. These are the ones that matter most.

I remember one job where the unit was showing a five-blink code but the ignitor looked fine, the gas valve was opening, and the flame was clearly visible. The flame sensor was reading zero microamps. I cleaned it, reconnected it, and it still read nothing. The problem turned out to be a cracked ground wire inside the harness where it connected to the board. The wire had fatigued from thermal cycling over eight years. I bypassed it temporarily with a jump wire to confirm, and the furnace ran normally after that. The manual doesn't mention this failure mode. It's one of those things you learn from dealing with enough of these units. The most common real failure I see on the Conquest 90 is the hot surface ignitor cracking. These are ceramic elements and they degrade with thermal stress. A typical lifespan is somewhere between five and eight years depending on cycle frequency. When it cracks, you get a four-blink code. The element may still glow, which tricks people into thinking it's fine. You need to check resistance across the ignitor terminals. A good ignitor reads somewhere in the 30 to 60 ohm range depending on the model. If it reads infinite, it's open. If it reads near zero, it's shorted. Either way, replace it. Don't try to run the furnace with a cracked ignitor. It will fail completely mid-cycle and leave you in lockout. Another issue that comes up regularly is the flame rectification signal being too weak. The flame sensor should read between 1 and 5 microamps DC during operation. If you're getting below 1 µA, the board won't confirm flame and will shut down the gas valve after the timeout period. Common causes are a dirty sensor, a poor ground connection, or excessive draft washing the flame away from the sensor. I've seen units in basements with negative house pressure that couldn't establish a stable flame because the exhaust draft was overpowering the burner. Installing a draft regulator or sealing obvious supply air leaks in the furnace room usually fixes it.

The pressure switch circuit is another frequent complaint area. The switch monitors whether the induced draft motor is creating proper manifold pressure. If the switch doesn't close within the expected time, the board faults out. A six-blink code points here. The plastic tubing that connects the pressure switch to the inducer housing can crack, fill with condensation, or become disconnected. Check the tubing first. It's cheap and takes two minutes. If the tube is clear and connected properly, test the pressure switch with a multimeter set to continuity. It should close when you apply suction to the switch port with a hand pump. If it doesn't close, the switch is bad. If it does close but the board still faults, the inducer motor may not be spinning fast enough or the heat exchanger passages could be partially blocked with debris. The dual-stage gas valve on these units is another component worth understanding. The valve has two coils for high and low fire. You can test each coil separately. Resistance on the low-fire coil should read around 60 to 70 ohms and the high-fire coil around 35 to 45 ohms. If either coil is open, the board can't modulate the flame and the furnace will lock out. I've also seen voltage drop at the valve terminals due to corroded connectors. The board might be sending the right signal but the voltage at the valve is too low to hold the solenoid open. Check for voltage at the valve while the furnace is calling for heat, not just at the board terminals. One thing the manual doesn't emphasize enough is the ignition transformer. It's a simple component but it fails more often than people expect. The primary winding should read around 10 to 15 ohms and the secondary, which produces the spark, should read several thousand ohms. If the secondary reads open, the spark won't jump the gap. I've had units where the spark looked strong from the outside but was actually arcing to ground internally because the insulator was cracked. Visually inspecting the transformer and the electrode assembly is worth doing before declaring the board bad.

The control board itself is the most expensive replacement part on this furnace, and it's also the part people jump to too quickly. Before replacing it, verify that the 24-volt transformer is putting out the correct voltage. It should read between 24 and 28 volts AC on the low-voltage terminals. Also check that the common wire is solid. A loose neutral on the board side can cause intermittent lockouts that look exactly like a bad board. I had one unit where the screw terminal for the white common wire was slightly loose, and the Furnace would run for a few cycles then quit randomly. Tightening that terminal solved the problem permanently. If you're working through a Conquest 90 Gas Furnace Troubleshooting Manual while diagnosing a unit, keep in mind the limitations of what it covers. The manual assumes standard residential installation conditions. It doesn't address units installed in conditioned spaces with tight clearances, units with modified venting, or aftermarket accessories that weren't officially listed. In those situations, the factory code definitions may not map directly to the actual problem. You need to fall back on basic electrical diagnostics: voltage checks, continuity tests, and component substitution. The manual is a starting point, not the final answer. The biggest pitfall I see technicians make is ignoring the heat exchanger condition. The Conquest 90 is a single-pass design with a U-shaped primary heat exchanger. Over time, condensation from the flue gases can cause corrosion, especially on the lower tubes. If the heat exchanger is cracked, no amount of troubleshooting the control system will fix the safety issues. The CO sensors in the draft system may trip intermittently, or the pressure switch may never seal properly because exhaust is escaping before it reaches the inducer. A flashlight and mirror inspection of the heat exchanger surfaces is something I do on every service call for these units, regardless of the initial fault code.

To download the official troubleshooting guide, search for the Bryant 391137 documentation package on the Bryant or Carrier technical document portal. Third-party HVAC sites host copies as well, but the manufacturer site is the most current source. Keep a printed copy in your truck. PDFs on phones are fine but slow to navigate when you're standing in a basement at 6 AM with a dead furnace and a cold house.