IBM PC AT Troubleshooting Guide

This guide covers diagnostic procedures for the IBM PC AT (5170), including POST audio and numeric error codes, hard drive faults, the well-documented 601 / CMOS battery / CMI HDD issues, and other known AT-specific problems.

IBM PC AT. Source: Wikimedia Commons.

Power-On Audio Beep Codes

Beep codes on the 5170 are produced by the BIOS through the PC speaker before video is initialised. Same coding scheme as on the 5150 and 5160.

IBM 5170 power-on audio codes

Pattern	Meaning	Stage
(no beep)	Power supply or motherboard not running — check 5 V/12 V, PSU minimum-load resistor, bank-0 RAM, CPU socket, system clock	Before POST
1 short	POST complete, system OK	End of POST
2 short	Numeric error code displayed; read the screen for the 1xx–1Fxx code	Any
1 long, 1 short	Motherboard speed test failed (Type 3 BIOS, motherboard speed outside expected window)	POST checkpoint
1 long, 2 short	Display adapter failure (MDA/CGA/EGA)	POST
1 long, 3 short	Enhanced graphics adapter (EGA) memory failure	POST
3 long	3270 keyboard test failure (variant boards only)	POST
Continuous beep	Power supply problem, motherboard problem, or stuck speaker	Any
Repeated short beeps	Power supply problem, often a shorted tantalum on +12 V or +5 V	Any

Numeric POST Error Codes

If the BIOS detects a fault during POST after video has initialised, it displays a numeric code on the screen and (usually) halts or prompts F1 to continue. The codes below are the IBM-specified set for the 5170 with original IBM BIOS; third-party BIOS replacements may use different codes for some classes.

1xx — System board

5170 1xx system board codes

Code	Meaning
101	Interrupt failure (8259A PIC)
102	Timer failure (8254 PIT channel 0)
103	Timer interrupt failure
104	Protected mode failure (80286 internal)
105	Last 8042 (keyboard controller) command not accepted
106	Converting logic test (post-protected-mode return)
107	Hot NMI test
108	Timer bus test
109	DMA test error (8237A)
110	System board parity error
111 / 112 / 113	Adapter card parity / I/O channel check
121	Unexpected hardware interrupt occurred
129	Cache error (later board steppings with cache support; unsupported CPU upgrade)
131	Cassette wrap test failed (legacy code, no cassette port on the AT)
161	CMOS configuration empty — battery failure or freshly fitted board
162	CMOS checksum error (adapter ID mismatch) — run SETUP
163	Time and date not set — run SETUP and re-enter
164	Memory size error — CMOS does not match installed RAM
166	Adapter card system board error
181	Bad CMOS configuration
199	User-indicated configuration not correct

2xx — Memory

5170 2xx memory codes

Code	Meaning
201	Memory test failed — the four hex digits to the left of "201" identify the failing byte's segment, the digit pair to the right identifies the failing bit position in that byte
202 / 203	Memory address line failure
215	64 KB memory module failure on a 16-bit memory expansion card

3xx — Keyboard

5170 3xx keyboard codes

Code	Meaning
301	Keyboard error (or stuck key) — the two hex digits before "301" are the scan code of the stuck key
302	Keyboard locked (5170 keylock is in the locked position) or keyboard test failure
303	Keyboard or 8042 keyboard controller error
304	Keyboard or 8042 keyboard controller error — CMOS does not match keyboard

4xx / 5xx — Display adapters

5170 4xx / 5xx display codes

Code	Meaning
401	Monochrome display adapter (MDA) failed
408	MDA display attribute failure
432	Parallel printer port test failed (on the MDA "monochrome and printer adapter")
501	Colour Graphics Adapter (CGA) failed
508	CGA display attribute failure

6xx — Floppy drive / controller

5170 6xx floppy codes

Code	Meaning
601	Diskette or controller adapter error (see "601 quirk" below)
602	Diskette boot record failure
606	Diskette verify error
607	Diskette write-protected
608	Bad diskette command
611	Time out (controller did not respond)
612 / 613	Bad NEC controller chip / DMA error

17xx — Hard drive

5170 17xx hard drive codes

Code	Meaning
1701	Fixed disk POST error — controller initialisation failed, or no drive ready
1702	Fixed disk adapter error
1703	Drive error
1704	Adapter or drive error
1780	Fixed disk 0 failure
1781	Fixed disk 1 failure

Other

• 7xx — Math coprocessor (80287) error
• 9xx — Parallel printer adapter error
• 10xx — Reserved (parallel adapter)
• 11xx / 12xx — Asynchronous communications (16450 UART) adapter errors
• 13xx — Game adapter error
• 14xx — Printer error
• 15xx — SDLC adapter error
• 18xx — I/O expansion unit error

Known Problems

The following AT-specific symptoms are documented at minuszerodegrees.net and confirmed by other long-term restorers.

601 error with 06/10/85 or 11/15/85 BIOS

With either of the 1985-dated BIOS revisions, removing the IBM Fixed Disk and Diskette Drive Adapter and substituting a third-party replacement can (not always) result in a 601 error at POST. There is something in those revisions that expects the IBM "combo" card. The error does not stop the boot; an F1 prompt allows boot to continue, although floppy operation may be compromised.

Workarounds:

• Use a replacement controller that the IBM BIOS recognises as the "combo" card: confirmed working replacements include the Western Digital WD1003A-WA2, WD1002-WA2 and WD1003-WA2.
• Use a patched IBM 5170 BIOS that disables the combo-card check.
• Use a non-IBM BIOS (AMI, Award, Phoenix). This is the cleanest workaround if 1.44 MB diskette boot support is desired.

The 01/10/84 BIOS does not produce 601 errors when the combo card is missing; this check was added in the second BIOS revision.

161 / 162 / 163 errors — CMOS battery failure

These three errors usually appear together when the external 6 V CMOS battery has run down. The MC146818 loses the CMOS configuration and the date/time. Replace the battery (see IBM PC AT Maintenance Guide) and re-run SETUP.

If 161/162/163 persist with a fresh battery, suspect a failed MC146818, a dirty or oxidised battery header, or a shorted CMOS line on the motherboard.

CMI 5616 / CMI 6426 hard drive failure

The 20 MB CMI 5616 drive (early 5170 models) and 30 MB CMI 6426 drive (later models) suffered well-documented reliability problems, leading to a PC Magazine article opening with the line "If you own an IBM PC AT and your hard disk hasn't crashed yet, don't worry — it probably will." If the drive does not spin up, makes a continuous click, or returns 1701/1780 errors, the drive itself is most likely faulty. Many enthusiasts retire the CMI drives in favour of a Seagate ST-225 (20 MB) or ST-251 (40 MB), or an MFM emulator such as XT-IDE / GoTek.

PSU will not start with diskette-only configuration

The 192 W AT PSU requires a minimum load on +12 V to start. Diskette-only ATs shipped with a 5 Ω 50 W resistor on the spare hard-drive power connector. If the resistor has been removed or has failed open, the PSU may not start. Verify the resistor is present and resistive (about 5 Ω across the +12 V/GND pins, fan off).

Floppy drive incompatibility

A 360 KB diskette written in the 1.2 MB drive cannot be reliably re-read in a 360 KB drive. The head track width of the 1.2 MB drive is half that of the 360 KB drive; the wider 360 KB read head picks up the half-track plus the remnants of any earlier track, producing garbled data. Use HD media in HD drives only.

Memory size error (164)

A 164 error means the BIOS POST detected an installed memory size that does not match the CMOS configuration. Run SETUP and re-enter the actual installed RAM size. If the size cannot be set correctly, suspect a failed RAM chip on the motherboard or memory expansion card — the 201 code (if present) will identify the failing address.

Slot keying and AT bus problems

Some XT-era 8-bit cards have a long tab that physically blocks the 16-bit AT extension. Cards designed for the AT are dual-tab: an 8-bit portion plus a shorter 16-bit extension. The slot itself does not damage incompatible cards mechanically, but the card will not seat fully and the system will refuse to boot.

No-Beep / No-Video Diagnostics

If the AT shows no signs of life:

1. Verify the PSU fan is turning. If not, check the wall power, the PSU mains lead, and the front-panel power switch.
2. With the PSU running, check +5 V at a free drive power connector. If absent, the PSU is faulty or one of its rails is shorted (likely a tantalum capacitor on the motherboard or an ISA card).
3. Pull all ISA cards except the video card. Re-test.
4. If still dead, swap in a known-good video card.
5. If still dead, reseat the CPU, the math coprocessor (if fitted), and the BIOS ROMs.
6. If still dead, suspect bank-0 RAM on the motherboard. The AT motherboard requires bank-0 RAM populated to POST.

Hard Drive Diagnostics

• 1701: Run SETUP and confirm the drive type matches the drive's label. The 5170 SETUP table has 15 drive types in the 01/10/84 BIOS, 22 in the 06/10/85 BIOS, and over 40 in the 11/15/85 BIOS.
• 1780/1781: Drive 0 or Drive 1 failure. Verify cable seating (pin 1 marked with a red stripe), drive power, drive ID jumpers (drive 0 vs drive 1), and that the drive is correctly terminated.
• Drive does not spin up: head stiction. Carefully twist the drive (with power off) to free the heads — this is a stop-gap, replace the drive afterwards.

Math Coprocessor (80287)

The 5170 socket accepts the Intel 80287 running at a divided clock. POST does not test the 80287 unless software explicitly invokes it; a missing or failed 80287 is reported as a 7xx error only by application software. Use IBM AT Advanced Diagnostics to test the 80287.

⚠️ Power-supply RIFA capacitor and tantalum shorts

Two age-related failures are near-universal on this era of IBM hardware:

• RIFA mains-filter capacitors in the power supply are metallised-paper parts that crack and fail short with age, producing acrid smoke shortly after power-on. Replace them pre-emptively with modern X2-class parts.
• Tantalum capacitors on the planar (system board) and on ISA cards fail short with age. A shorted tantalum will prevent the power supply from starting (dead machine, PSU protection latched) — look for a cracked or discoloured tantalum and lift suspect ones to find the short.

IBM PC/XT switching supplies also need a minimum load to start, so a bare supply on the bench may not run without a dummy load.

⚠️ CMOS / RTC battery

This machine keeps its configuration in battery-backed CMOS, and the battery is a common failure. On AT-class boards the clock/CMOS is often a Dallas DS1287/DS12887 module with the cell sealed inside; it lasts about ten years and then dies, giving 161 / 163 CMOS and clock errors at POST (and sometimes spurious floppy-drive errors). PS/2 planars use a rechargeable barrel or pack battery that leaks and corrodes the board. Replace a dead Dallas module (or rework it with an external coin cell), and on a leaking planar battery remove it and clean the corrosion before it eats the traces.

Related Pages

• IBM PC AT (5170)
• IBM PC AT Maintenance Guide
• IBM PC AT Capacitor Replacement Guide
• IBM PC XT Troubleshooting Guide — preceding model
• IBM PC (5150) Troubleshooting Guide — preceding model

References

• IBM 5170 — POST checkpoint codes, minuszerodegrees.net.
• IBM 5170 — BIOS Revisions, minuszerodegrees.net (601 error notes for the 06/10/85 and 11/15/85 BIOS).
• HelpPC — Diagnostic Codes. Reference for the 1xx–18xx numeric error code meanings.
• IBM, Personal Computer AT — Hardware Maintenance and Service (IBM service manual; multiple revisions 1984–1987).
• IBM, Personal Computer AT — Technical Reference (1502243, March 1984; 6280070, September 1985).