Safety first, because this one is mains

The P1S heatbed is not a low-voltage part. Bambu state plainly in their own troubleshooting article that the heatbed is heated directly from the mains supply, that there is a high-voltage hazard, and that fault-finding must be carried out with the power off.

That is not boilerplate on this machine. Unplug the printer at the wall — not just the front switch — before opening anything, and treat the AC power board and the bed’s power terminals as live-until-proven-otherwise. If you are not comfortable measuring resistance on a mains-fed heater with the machine isolated, this is a reasonable point to stop and raise a ticket instead.

The symptom pattern

The print starts, the printer moves into its preparation sequence, and the bed temperature simply does not climb. Sometimes the print sits waiting indefinitely; sometimes it errors out. The nozzle may heat perfectly normally, which is what makes owners suspect a firmware bug rather than a hardware fault.

The one reading that splits the fault

Before touching a screwdriver, look at what the printer says the bed temperature is. Bambu’s own procedure branches on exactly this, and it is the most useful thirty seconds you can spend.

Bed reads 0. The printer is not getting a temperature at all, so it never commands heat. This is a sensing fault, and it is the cheaper branch. The suspects are the heatbed NTC connector and the cable running from the heatbed to the MC board — a two-pin plug for the NTC, plus the main connector. Both are fixed in place with white silicone gel, which matters: a connector held in soft gel can lift a pin without looking obviously wrong. Bambu’s advice is to press them home again if there is no obvious tilt or withdrawn pin, and to soften the gel with a hair dryer first if a connector genuinely has to be disconnected.

Bed reads something plausible but never heats. The sensor is telling the truth and the heat is not arriving. That points at the mains supply path to the bed, and it is the more expensive branch.

Working through the power-side branch

With the printer isolated, the bed’s own resistance is the measurement that decides everything. Bambu give a specific window: between 40 and 60 ohms across the two heatbed terminals is normal. Outside that range the heatbed has burnt components or an open circuit and needs replacing — no further testing required.

If the bed measures in range, the same check applies to the heatbed power cable, measured after removing its two blue silicone sleeves. Bambu add a caution worth repeating: put those sleeves back afterwards, because they are what keeps the plugs insulated.

Then the connections. The cable between the MC board and the AC power board is checked for looseness and skew in the same way as the sensor side, and poor contact there is a real cause rather than a formality.

The fix

If the heatbed or its power cable measures outside 40–60 ohms, that part is the failure and gets replaced.

If the bed, the cable and every connector check out, Bambu’s procedure reaches a conclusion rather than a shrug: the AC power board is faulty and must be replaced. Visible damage on the board confirms it without further testing — and their article adds a sensible warning that where one of the heatbed and the AC board is found burnt, the other should be inspected at the same time, because these two fail together.

Verdict

Very much worth diagnosing before spending. The two outcomes at the end of this — a heatbed or an AC power board — are among the dearer parts in the machine, and the resistance test is a five-minute check that tells you which one you need rather than buying both hopefully. A re-seated connector, which is a genuine possibility on the sensor branch, costs nothing at all.

Both parts are user-replaceable and Bambu publish the replacement procedures, so this is not a printer to write off. Price the heatbed and the AC board before ordering, and if the diagnosis stalls between the two, Bambu’s support ticket route exists for exactly that.