Pulse oximeters

Nellcor N-20: troubleshooting and error codes

The ServiceForge knowledge base holds 12 documented symptoms for the Nellcor N-20, with 25 ranked causes, 26 diagnostic checks with test points and expected values, and 26 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Pulse Search indicator appears for more than 5-10 seconds patient safety

Pulse Search indicator remains on longer than normal, indicating difficulty finding a pulse.

Ranked causes

1. Sensor improperly positioned (common)
2. Incorrect sensor in use (common)
3. Perfusion too low (occasional)
4. Foreign material on sensor LEDs or photodetector (occasional)
5. Patient or environmental motion interfering (occasional)
6. Sensor too tight or excessive illumination (occasional)
7. DB-9 sensor connector broken (rare)

First check

Verify sensor placement

Test point: Sensor application site
Expected: Sensor correctly positioned per directions for use

5 more checks with pass and fail actions, 7 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

No response to Measure button

Pressing the Measure button produces no response from the oximeter.

Ranked causes

1. Battery access door not properly latched (common)
2. Batteries discharged (common)
3. Batteries installed incorrectly (occasional)
4. Battery contacts not making electrical connection (occasional)
5. Fuse F1 open on auxiliary PCB (occasional)
6. Dust under Measure button causing contact loss (rare)

First check

Verify battery access door is properly latched

Test point: Access door
Expected: Door fully latched and secure

5 more checks with pass and fail actions, 6 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Pulse Search indicator appears after successful measurements patient safety

After making successful measurements, Pulse Search indicator suddenly appears, indicating loss of signal.

Ranked causes

1. Patient perfusion too low (common)
2. Patient motion interfering with pulse tracking (common)
3. Sensor too tight or excessive illumination (occasional)

First check

Assess patient perfusion status

Test point: Patient vital signs and limb perfusion
Expected: Adequate and stable perfusion

2 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Performance verification after repair

Required materials and test setup for N-20/N-20P pulse oximeter post-repair validation including display backlight, battery, and self-test verification

Ranked causes

1. Post-repair verification required (common)

First check

Main backlight illumination in low light

Test point: Oxygen Saturation and Pulse Rate display, 14-segment pulse rate amplitude indicator
Expected: Backlight illuminates in darkened room

2 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Dashes appear in the display

Display shows dashes (– – –) instead of pulse oximetry readings.

Ranked causes

1. Sensor not connected to instrument (common)

First check

Verify sensor connection

Test point: Sensor DB-9 connector and instrument input
Expected: Sensor firmly connected to instrument; no extension cable

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Display backlight not illuminating

One or more sections of electroluminescent backlight fail to illuminate under low light conditions or during power-on

Ranked causes

1. Backlight component failure or connection issue (unknown)

First check

Ambient light detector function

Test point: Circular window in top right corner of display
Expected: Main section backlight activates in low light conditions

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Error message with number displayed

Err followed by a number appears on display; specific error code not identified at this stage

Ranked causes

1. Unknown error code (unknown)

First check

Note error number on display

Test point: LCD display field
Expected: Err followed by numeric code

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Paper mechanism jams during loading or printing caution

Paper stuck between print head and printer mechanism or in paper path; difficulty advancing or removing paper

Ranked causes

1. Paper caught in printer mechanism (occasional)

First check

Assess print head position

Test point: Print head carriage location
Expected: Print head at home position (right side)

3 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Printer fails to operate

Printer is non-functional; N-20P model only

Ranked causes

1. Fuse F2 on auxiliary PCB open (occasional)

First check

Inspect fuse F2 on auxiliary PCB

Test point: Auxiliary PCB fuse F2
Expected: Fuse intact and conducting

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Printer not operational while monitor continues measuring

Paper jam or print head not at home position; patient measurements continue unaffected

Ranked causes

1. Paper jammed in printer mechanism (common)

First check

Inspect paper compartment for jammed paper

Test point: Paper path in printer
Expected: No paper obstruction; print head at home position

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Printer paper advances but no printing occurs

Paper moves through mechanism but thermal print head does not mark; N-20P only

Ranked causes

1. Thermal paper improperly loaded (common)

First check

Verify thermal paper orientation

Test point: Paper loaded in printer compartment
Expected: Thermal side facing print head; paper seated in slot correctly

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Time or date incorrect on display

Clock shows wrong time or date; N-20P model only

Ranked causes

1. Real-time clock (RTC) battery exhausted (occasional)

First check

Verify displayed time and date

Test point: LCD display after startup
Expected: Correct time and date matching current date/time

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Error codes

Code	Meaning	Trigger	Severity
`101`	Error in internal RAM registers test	Power-on self-test or performance test detects internal RAM register failure	critical
`102`	Error in zero register test	Power-on self-test or performance test detects zero register failure	critical
`103`	Error in register contents clearing test	Power-on self-test or performance test detects register clearing failure	critical
`104`	Error in register contents increment test	Power-on self-test or performance test detects register increment failure	critical
`105`	Error in register contents decrement test	Power-on self-test or performance test detects register decrement failure	critical
`106`	Errors in logical operations test	Power-on self-test or performance test detects logical operation failure	critical
`110`	Error in exchange test	Power-on self-test or performance test detects exchange operation failure	critical
`111`	Error in timer tests	Power-on self-test or performance test detects timer failure	critical
`112`	Error in window select register test	Power-on self-test or performance test detects window select register failure	critical
`113`	Errors in stack manipulation test	Power-on self-test or performance test detects stack manipulation failure	critical
`115`	Errors in CPU flags test	Power-on self-test or performance test detects CPU flags failure	critical
`118`	Error in interrupt pending register test	Power-on self-test or performance test detects interrupt pending register failure	critical
`119`	Error in program counter test	Power-on self-test or performance test detects program counter failure	critical
`120`	Error in CPU serial port test	Power-on self-test or performance test detects serial port failure	critical
`121`	Error in pulse width modulation register test	Power-on self-test or performance test detects PWM register failure	critical
`122`	Error in A/D register test	Power-on self-test or performance test detects A/D register failure	critical
`123`	Error in addressing modes test	Power-on self-test or performance test detects addressing mode failure	critical
`124`	Error in high speed input register test	Power-on self-test or performance test detects high speed input register failure	critical
`125`	Error in content addressable memory test	Power-on self-test or performance test detects CAM failure	critical
`201`	Errors in external RAM test	Power-on self-test or performance test detects external RAM failure	critical
`301`	Error in PROM test	Power-on self-test or performance test detects PROM failure	critical
`401`	Errors in I/O port test	Power-on self-test or performance test detects I/O port failure	critical
`601`	Failure of real-time clock	Power-on self-test or performance test detects real-time clock failure	critical
`602`	Errors in real-time clock	Power-on self-test or performance test detects RTC error	critical
`701`	Errors in watchdog-timer	Power-on self-test or performance test detects watchdog-timer failure	critical
`Pr Err`	Printer error condition	Printer detects error during operation	error

Likely causes and actions for each code are in ServiceForge. Connect ServiceForge to your AI

Community

0 fixes logged by technicians through their AI, with outcomes, for this model.

ServiceForge knowledge base record. Field-unverified unless technicians confirmed it. A teaching summary of the record, not a service manual. Export 2026-10-14.