Epson L1800/1390 Printhead Multimeter Test: What "Low Ohms" (Like 575Ω vs 875Ω) Really Means-and When a Head Is Truly Bad

Question: My Epson L1800 printhead measures much lower than the chart on the 9-pin cable-575Ω instead of ~875Ω on pin 2. If the pins that should be "OL" still read OL, does that mean the printhead is bad? I'm still a beginner and don't want to burn the mainboard.

Answer: 

Now, to your measurement: seeing ~575Ω on pin 2 of an Epson L1800/1390-style head is actually pretty common on a worn or aging head, and by itself it does not automatically mean the printhead is "bad." It does mean you should treat the situation cautiously, and you're absolutely right to worry about protecting the mainboard.

1) Why 575Ω (instead of ~875Ω) can happen

Those resistance charts are typically "healthy head" reference ranges, but real-world printheads drift over time due to:

• Heat cycles and age (normal wear)

• Ink residue / moisture paths that slightly change readings

• Internal piezo/trace deterioration that lowers resistance in one channel group

So a lower-than-chart value (like 575Ω vs 875Ω) often suggests wear, not an instant death sentence.

2) What "OL" readings actually tell you (and what they don't)

When a chart says certain pin pairs should show OL (open line), that's usually checking for isolation-meaning those circuits are not supposed to be connected.

• If you get OL where OL is expected, that's generally a good sign (no obvious short between those sections).

• But OL being correct does not guarantee the head is good, because a head can be electrically "not shorted" and still be:

  • partially clogged,

  • electrically weak (failing under load),

  • or have a damaged nozzle bank that doesn't show up as a simple short/open fault.

Think of it like checking a light bulb with a meter: it can show continuity but still fail when powered.

3) The real danger pattern (the one that can damage the mainboard)

The biggest "red flag" readings are:

• Very low resistance compared to typical values (for example, readings that plunge dramatically)

• Near-short readings (often much lower than normal range)

• Inconsistent readings across pin groups that should be similar

A worn head might read lower, but if you're seeing something dramatically low (or wildly inconsistent compared to the other sections), that's when you should be extra careful-because that can increase current draw and risk damaging the driver circuitry on the mainboard.

4) How to judge your head properly (not just one pin)

Your best next step is to evaluate the entire pattern across all relevant pins, not only pin 2.

Here's what you're looking for:

• Are the "normal ohm" pins all in the same neighborhood?
  If most are reasonably consistent and only one is moderately low, that points more toward wear rather than catastrophic failure.

• Do you have any unexpected continuity where OL should be?
  That suggests a short/leak path, which is risky.

• Do readings change a lot if you retest?
  If the value jumps around, suspect contamination, moisture, probing issues, or an unstable internal fault.

5) Beginner-safe multimeter tips (to avoid false readings)

A lot of "bad head" scares come from testing issues. A few tips:

• Unplug power completely and give the printer a moment before testing.

• Disconnect the printhead cables when measuring the head (don't measure through the board).

• Use a stable range (typically 2kΩ / 20kΩ range depending on meter).

• Don't press hard or slip pins-a slipped probe can hit adjacent pins and cause confusion.

• Clean the contacts (lightly) if you suspect residue is affecting readings.

6) So... is your printhead bad?

Based on what you shared:

• 575Ω on pin 2 alone is not enough to declare the printhead bad.

• If your OL readings match what the chart expects, that's encouraging from a "not shorted" standpoint.

• However, the lower resistance suggests wear, and you should confirm the full set of readings across all pins/groups before deciding.

If you also have symptoms (missing nozzle sections that won't recover, recurring cleaning cycles with no improvement, overheating behavior, repeated fuse/board issues, or frequent head driver failures), then the electrical reading becomes more meaningful in context.

Addressing printer issues can be complicated because so much of it is hands-on and depends on what we see physically in the unit. Because of that, we're not able to provide remote troubleshooting or step-by-step repair support. We do offer in-person evaluation and repair through our local diagnostic facility: Printer Repair Service (https://bchtechnologies.com/printer-repair-service). Due to high demand, we operate first-come, first-served, and it can take a few weeks before we're able to take a drop-off. Our services can be set up to repair either the whole printer or specific parts, with clear instructions on how to proceed. That said, we understand our rates may not be the most economical-so we strongly recommend self-help through online research. A great starting point is YouTube, especially our channel homepage: BCH Technologies on YouTube (https://youtube.com/@bchtechnologies). You can search our videos using the search icon next to "About" on the right side of the menu bar. I receive dozens of questions daily asking for a video on a specific topic, and since we've created videos over the past nine years, it's hard to remember every single one-so YouTube search is the fastest method, and it may also suggest helpful videos from other creators.

Thanks again for reaching out and for supporting our work-please don't hesitate to keep the questions coming, and be proud you're learning this stuff the right way (carefully and safely).