Garmin GPSMAP 421s Manual

The Garmin GPSMAP 421s is a 4-inch color chartplotter with built-in BlueChart g2 U.S. coastal charts, a high-sensitivity GPS receiver, and sonar capabilities. It features a sunlight-readable display, waterproof design (IPX7), NMEA 0183 and NMEA 2000 connectivity, microSD card slot for additional charts, and support for Garmin marine network accessories. Below are key sections for safety, features, installation, setup, operation, and troubleshooting.

Features at a Glance

Key components: 4-inch color TFT display, built-in GPS antenna, sonar transducer port, microSD card slot, power/data cable connector, and waterproof housing.

Feature	Description
Display	4-inch diagonal, 480 x 272 pixels, sunlight-readable, color TFT
GPS Receiver	High-sensitivity, WAAS-enabled
Preloaded Charts	BlueChart g2 U.S. coastal charts
Sonar	Supports 50/77/200 kHz frequencies (transducer dependent)
Waterproof Rating	IPX7 (submersible 1 meter for 30 minutes)
Connectivity	NMEA 0183, NMEA 2000, microSD card slot
Power Input	10–35 V DC
Mounting	Flush mount or bracket mount options
Buttons	POWER, PAGE, ZOOM IN/OUT, ENTER/MARK, ROCKER key
Audio Alarms	Configurable for anchor drag, waypoint arrival, etc.

Installation

Unpack and inspect contents. Choose mounting location with clear view of sky for GPS reception.

1. Mount the unit: Use included bracket or flush mount kit. Ensure surface is flat and strong.
2. Connect power: Route power cable to battery. Connect red wire to positive (+), black wire to negative (-). Install inline fuse (3 A) near battery.
3. Connect transducer: Route transducer cable to transom or through-hull location. Connect to SONAR port.
4. Connect NMEA devices: If using, connect NMEA 0183 or NMEA 2000 cables per wiring diagrams.
5. Secure cables: Use cable ties to prevent strain and water ingress.

WARNING! Disconnect battery before making electrical connections. Ensure all connections are waterproofed.

First-Time Setup

Power on unit; follow initial prompts.

1. Select language: Choose from available options (default is English).
2. Configure units: Set distance, speed, depth, and temperature units (e.g., nautical miles, knots, feet, °F).
3. Calibrate compass (if applicable): Follow on-screen instructions.
4. Set time: Choose automatic GPS time or set manually.
5. Acquire GPS signal: Move to open area; wait for 3D fix (typically under 45 seconds).

CAUTION! Do not power unit without proper voltage (10–35 V DC).

Specifications

• Display Size: 4 inches diagonal.
• Display Resolution: 480 x 272 pixels.
• GPS: High-sensitivity, WAAS-enabled, < 3 meters accuracy.
• Charts: Preloaded BlueChart g2 U.S. coastal charts; accepts BlueChart g2 Vision cards.
• Sonar: Frequency support depends on transducer (50/77/200 kHz). Depth capability up to 2500 ft (762 m).
• Power: 10–35 V DC, 3 A max.
• Waterproof: IPX7.
• Operating Temp: -15° to 55°C (5° to 131°F).
• Storage Temp: -40° to 85°C (-40° to 185°F).
• Dimensions: 15.2 x 10.9 x 6.6 cm (6.0 x 4.3 x 2.6 in).

Controls and Settings

Access menus via PAGE button and ROCKER key. Front Panel: POWER, PAGE, ZOOM IN/OUT, ENTER/MARK, ROCKER key (up/down/left/right/enter).

Main Pages: Chart, Sonar, Compass, Highway, Data. Use PAGE to cycle.

Settings: System (Units, Time, Alarms, NMEA), Chart (Orientation, Detail, Tracks), Sonar (Frequency, Range, Gain, Colorline), Navigation (Waypoints, Routes).

Waypoints: Press ENTER/MARK to save current location. Edit name, symbol, depth.

Routes: Create by placing waypoints on chart; navigate route using GOTO.

Connecting Devices

NMEA 0183: Connect to external devices (e.g., VHF radio, autopilot) using dedicated TX/RX wires. NMEA 2000: Connect to compatible network using micro-C connector. microSD: Insert card for additional charts or software updates.

1. Turn off power before connecting/disconnecting cables.
2. Match wire colors per manual diagrams.
3. Use waterproof heat-shrink tubing on splices.
4. Insert microSD card with label facing up.

Tip: Test NMEA connections with data fields (e.g., COG from autopilot).

Using Charts & Navigation

Preloaded charts show depth contours, navaids, marinas, and points of interest.

1. View chart: Press PAGE until Chart page appears. Zoom with ZOOM IN/OUT.
2. Set waypoint: Press ENTER/MARK at cursor or current position.
3. Navigate to waypoint: Select waypoint from list, choose GOTO.
4. Follow route: Activate route from Routes list; follow course line.
5. Use data fields: Add speed, depth, distance to waypoint, etc., to screen.
6. Track recording: Enable in Tracks menu to record path.
7. Man Overboard (MOB): Press and hold ENTER/MARK to mark MOB location and initiate navigation back.

WARNING! Always compare electronic charts to paper charts and local knowledge. GPS accuracy can be affected by conditions.

Sonar Features

Shows bottom contour, fish arches, and structure. Press PAGE to switch to Sonar page.

1. Adjust range: Use ROCKER key to set depth range manually or choose Auto.
2. Adjust gain: Increase to see weaker returns; decrease to reduce clutter.
3. Set frequency: Select 50 kHz for deeper water, 200 kHz for better detail in shallow water.
4. Use A-scope: Shows real-time sonar returns on right side.
5. Set alarms: Depth alarm, fish alarm, temperature alarm.

Note: Sonar performance depends on transducer installation and water conditions.

Care & Cleaning

Disconnect power before cleaning. Use fresh water and mild soap on housing. Wipe screen with soft, damp cloth. Do not use abrasive cleaners or solvents.

CAUTION! Do not spray water directly into ports. Ensure unit is dry before reconnecting power.

Regulatory Information

FCC Compliance: This device complies with part 15 of the FCC Rules. Operation is subject to condition that device does not cause harmful interference. IC Compliance: Complies with Industry Canada licence-exempt RSS standard(s). Copyright © Garmin Ltd. or its subsidiaries. BlueChart and GPSMAP are trademarks of Garmin Ltd. For support, visit www.garmin.com/support or contact Garmin Product Support at 913-397-8200 or 1-800-800-1020.

Troubleshooting

Symptom	Possible Cause	Corrective Action
No power	Blown fuse, loose connection	Check inline fuse; verify battery connections; ensure voltage 10–35 V DC.
No GPS signal	Obstructed view, poor satellite geometry	Move to open area; ensure no metal obstructions above unit; wait several minutes.
No sonar display	Transducer not connected, wrong source	Check transducer connection; select Sonar page; ensure transducer is submerged.
Chart not detailed	Zoom level, chart region	Zoom in for more detail; ensure microSD card with detailed charts is inserted.
Buttons not responding	Unit locked, moisture	Press and hold POWER to restart; ensure unit is dry.
Incorrect depth reading	Transducer interference, air bubbles	Adjust transducer location away from hull turbulence; check for fouling.

Reset: Press and hold POWER for 10 seconds to force restart.

Software Update: Download latest software from Garmin website; load onto microSD card; insert and power on unit.

Service: 1-800-800-1020; www.garmin.com/support