Based on our service history from 2023-2025, we've identified key error codes and issues experienced by customers with water heaters from the A.O. Smith BTH 300 series. Recognizing these patterns allows us to provide faster, more accurate, and more cost-effective solutions for your commercial hot water needs. Over the past few years, we serviced at least 58 A.O. Smith BTH 120-250 300/301 Series units. A summary of this data is reflected in the table entitled "BTH 300 Series Service by the Numbers."

We tracked this information by entering creating an automation that tagged each service call with the water heater's model number. However, the data shared in this article does not reflect on all models from this series.

Top 5 Common Issues on A.O. Smith BTH 120-250, 300 Series Heaters

1. Ignition Failures (B2-70 Fault Code)

The most common issue our technicians encounter during A.O. Smith BTH 120-250, 300 Series repair calls is the B2-70: Ignition Failure error code. In our review of 58 service calls, ignition-related failures accounted for 27.5% of all visits (16 out of 58)—making this the single most frequent service issue we see in the field. The problem was especially prevalent in two models: the BTH 120-300, which represented 9 of the 16 ignition failures, and the BTH 199-300, which accounted for 7 of the 16 ignition failures.

The B2-70 ignition failure typically indicates that the burner did not light or the flame was not detected after multiple ignition attempts. In most cases, our technicians resolved the issue by replacing the ignitor or installing a new rectifier assembly, both of which are critical components of the ignition and flame-sensing circuit (Source: A.O. Smith Manual). However, these were not the only causes—or solutions—identified.

Further investigation across multiple sites revealed several contributing factors, including worn igniters, dirty flame rods, moisture in the reference tubes, and occasionally a faulty gas control valve (GCV). Each of these issues can disrupt the ignition sequence or prevent the control board from verifying flame presence, triggering a B2-70 fault.

To prevent future ignition-related shutdowns, we recommend including a flame rod cleaning and inspection in every annual maintenance cycle. Technicians should also check the condensate exhaust elbow and reference tubing for moisture or blockage, as trapped water can interfere with the ignition control circuit. Proactive maintenance not only minimizes ignition errors but also extends the overall lifespan and reliability of your A.O. Smith BTH 120-250, 300 Series water heater.

2. Blower Feedback or Speed Errors (BE-1 Fault Code)

3. Client Misdiagnosed "Leaking" Water Heaters: Leaking Condensate Drain Lines, Leaking T&P Valves

Among our service calls for “leaking” A.O. Smith BTH 120-250, 300 Series water heaters, we documented six cases where clients reported a leak. Of these, only one case involved a BTH 120-300, and in that instance, the unit actually had a true leak. The remaining five cases involved BTH 199-300 units, and in every instance, the reported leaks were not caused by the water heater itself.

In these five misdiagnosed cases: two were due to leaking T&P valves, two were caused by leaking condensate drain lines near the heater, and one involved a leaking pipe above the water heater, which dripped down into the blower assembly and was mistaken for a leak coming from the heater itself.

To prevent misdiagnosis and corrosion damage, technicians should inspect condensate lines and elbows, and ensure correct drain routing. Regular inspection of the vent and condensate system is a key step in the A.O. Smith BTH 300 Series preventative maintenance, helping to ensure safe operation, reduce unnecessary service calls, and extend the life of the unit.

4. Temperature Probe Faults & Failures

Temperature probe faults are a less frequent but critical issue on A.O. Smith BTH 120-250, 300 Series water heaters, and can cause the unit to enter shutdown or standby mode if the control board detects an open or shorted circuit. In our documented service calls, there were three cases involving temperature probe errors, all indirectly highlighting the importance of regular inspection and preventative maintenance.

Case 1 – BTH 120-300 (A5-401: High Temperature Limit Exceeded): The top temperature probe was found to have an open circuit, negatively impacted by sediment buildup. The technician replaced the probe from truck stock, restoring proper operation.

Case 2 – BTH 199-300 (A5-401: High Temperature Limit Exceeded): This customer's unit had never been maintained, and the temperature probe was no longer accurately sensing water temperature due to extreme wear and buildup. After replacing the probe, additional ignition issues were uncovered, requiring replacement of the flame rod and ignitor.

Case 3 – BTH 199-300 (48-0: Possible Temperature Probe Issue): In this case, the lower temperature probe had been physically bumped and its clip broken. This is a common problem in restaurant environments where carts or mop buckets may bump into the lower portion of the unit without having an immediate negative impact. Following A.O. Smith recommendations, the technician:

• Verified that the probe cable was plugged into the CCB and connector
• Measured resistance to check for an open or shorted circuit
• Inspected the probe and cable for damage
• Ultimately, the solution was to replace the lower temperature probe and harness

Preventive Tips: Temperature probes should be checked at least annually for tight connections, physical damage, and potential build-up. Replacing the temperature probe harness is a necessity if the clips or harness itself are damaged. This can prevent unexpected shutdowns and extend the life of the water heater. Regular inspection of both the upper and lower temperature probes is another important step in A.O. Smith BTH 120-250, 300 Series preventative maintenance, ensuring accurate temperature sensing and reliable operation.

5. CCB (Central Control Board) or Software Version Faults

Faults related to the Central Control Board (CCB) or outdated software versions are another notable source of issues on A.O. Smith BTH 120-250, 300 Series water heaters. In our documented service calls, we observed four cases of hardware failure across these units, all traced to faulty CCBs. Two cases involved BTH 120-300 units and another two cases involved BTH 199-300 units with hardware failures.

In every instance, the solution was to replace the CCB with the most current software version, which restored proper system operation and resolved the hardware fault codes.

Additionally, we encountered a fifth case where a customer reported a B2-70 ignition failure code. On site investigation revealed multiple prior errors in the unit’s error history, and the CCB display was not functioning properly, preventing accurate diagnostics. In this case, the technician replaced both the gas control valve (GCV) and the CCB from truck stock, which corrected the ignition errors and restored normal operation.

Preventive Tip: Knowing your unit's CCB's software version (and ensuring the software version is the most up to date) can prevent recurring hardware failures, display malfunctions, and other related errors. For units showing repeated or multiple hardware error codes, a faulty CCB should be considered a potential root cause, especially if other components have already been checked. We recommend adding CCB inspection as part of A.O. Smith BTH 120-250, 300 Series preventative maintenance to ensure.

The A.O. Smith BTH 120-250, 300 series are high efficiency commercial gas water heaters. These commercial water heaters (part of the high efficient Cyclone Mxi series) deliver strong performance when properly serviced. Routine BTH Series maintenance — such as cleaning the flame sensor, inspecting condensate lines, and checking ignition components — prevents costly downtime and ignition faults. Here are some key takeaways from our research:

1. Neglecting annual maintenance has been the #1 cause of A.O. Smith BTH repair calls for our clients.
2. Error codes are hints, but they do not always tell the full story about what's happening with your A.O. Smith BTH 300 series water heater (or any other water heater for that matter)...
3. Water on the floor doesn’t always mean a leaking water heater tank — blocked exhaust elbows or cracked condensate lines are common culprits. Regularly inspect the condensate drain and vent slope to prevent corrosion or shutdowns. This simple maintenance step can prevent major A.O. Smith BTH repair costs.
4. Proactive maintenance reduces repair frequency and extends equipment life. Maintenance improves reliability and helps avoid unexpected ignition failure and no blower feedback faults.