IBM PC AT Troubleshooting Guide - Revision history

Josh: Add representative photo (Wikimedia Commons, attributed on file page)

2026-07-16T12:06:30Z

Add representative photo (Wikimedia Commons, attributed on file page)

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			[[File:IBM PC AT (photo).jpg\|thumb\|right\|300px\|IBM PC AT. Source: Wikimedia Commons.]]
	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.		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.

Josh: Expand troubleshooting: RIFA/tantalum + Dallas/planar battery (161/163) + POST codes; cited

2026-07-15T23:16:54Z

Expand troubleshooting: RIFA/tantalum + Dallas/planar battery (161/163) + POST codes; cited

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	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.		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.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>
			* '''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.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>

			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.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>

			== ⚠️ 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.<ref name="ibm_batt">[https://www.classic-computers.org.nz/blog/2009-10-10-renovating-a-dallas-battery-chip.htm Fixing a Flat Dallas DS1287 RTC], Classic Computers; and [https://www.ardent-tool.com/misc/Dallas_Rework.html Reworking Dallas RTC Modules], Ardent Tool. Source for the Dallas DS1287/DS12887 internal-battery death (161/163 CMOS errors) and the leaking planar battery.</ref>

			== References ==

			<references />

	== Related Pages ==		== Related Pages ==

Josh: Create/rewrite IBM PC AT (5170) and IBM PCjr (4860) page set with verified facts, real images, and embedded content per RTC style guide

2026-05-21T10:18:32Z

Create/rewrite IBM PC AT (5170) and IBM PCjr (4860) page set with verified facts, real images, and embedded content per RTC style guide

New page

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.

== 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.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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:
# Verify the PSU fan is turning. If not, check the wall power, the PSU mains lead, and the front-panel power switch.
# 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).
# Pull all ISA cards except the video card. Re-test.
# If still dead, swap in a known-good video card.
# If still dead, reseat the CPU, the math coprocessor (if fitted), and the BIOS ROMs.
# 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.

== 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 ==

* [https://minuszerodegrees.net/5170/post_errors/5170_post_codes.htm IBM 5170 — POST checkpoint codes], minuszerodegrees.net.
* [https://minuszerodegrees.net/5170/bios/5170_bios_revisions.htm IBM 5170 — BIOS Revisions], minuszerodegrees.net (601 error notes for the 06/10/85 and 11/15/85 BIOS).
* [https://helppc.netcore2k.net/hardware/diagnostic-codes 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).

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