List of codes and their meanings on Oyster sensors

00 - Valid Ticket
01 - UTS Gate Fault (Invalid Tables) / Bad Data
02 - Wrong Type/Direction of use for this type of ticket
03 - Wrong type of National Rail only (No TfL/LU validity)
04 - UTS Gate Fault (Check Table Error)
05 - Technician signed onto machine (Gate in test mode)
06 - Wrong mode or test ticket (Not Valid for travel)
07 - Code Unreadable (Usually when ticket is upside down)
08 - Recoding Error, gate failed to correctly recode ticket. Replace the ticket.
09 - Ticket damaged, usually bent demagnetised or corrupted ticket - replace.
10 - Carnet Validated (Carnet Validator Only)
11 - Out of date, ticket expired.
12 - Not valid at this time (e.g. OPTC before 0930, Freedom pass before 0900)
13 - Additional Fare Payable (Under value, additonal fare due)
14 - Not Valid, and is not excessable
15 - No onward validity
16 - Outside zone - ticket invalid station
17 - Not issued here - platform ticket from another station
18 - Not valid here - Not valid at this station (Station Permit at wrong station)
19 - Start date in future (Not valid yet)
20 - Oyster card expired
21 - Ticket already used for entry (Double entry)
22 - Ticket already used for exit (Doube exit)
23 - Carnet not validated on entry
24 - Has season ticket but out of zone with insufficient Pre-Pay to cover extension
25 - Unstarted journey (Not validated on entry)
26 - Entry and exit at same station (Customer possibly decided not to travel)
27 - Pre Pay not enabled
28 - Pre Pay stopped (PAYG capacity disabled)
29 - Season ticket disabled
30 - Oyster Card stopped (Hot listed Oyster card)
31 - Invalid from OSI (Out of Station interchange)
32 - Invalid Into OSI (Out of Station interchange)
33 - Illogical Interchange through gates [1]
34 - Exit not allowed - Pre Pay not vaidated on journey
35 - Out of money! - Insufficient Pre Pay
36 - Out of money! - No valid season ticket
37 - Unknown
38 - Unknown
39 - Unknown
40 - Unknown
41 - Ticket used three times in quick succession (Zig Zag) at same station (entry-exit-entry or exit-entry-exit or purchase-entry-exit)
42 - Double use in one direction (Pass back)
43-50 (Unused/Unknown)
51 - Already used for 1 journey (single) or 2 journeys (return)
52 - Pass Reported Lost or Stolen [2] (Hot listed)
53 - Permit Reported Lost or Stolen (Black listed) [2]
54 - Cancelled magnetic ticket (Cancelled due to use before start date) [Possible dumb bell]
55 - Already used for journey
56 - Unknown/Unused
57 - Station not covered on period ticket
58-60 Unknown/Unused
61 - Too long spent making interchange
62 - Too long spent on journey (carnets only)
63 - Too long on platform (platform tickets only)
64 - Too long on excess (Fare-Paid tickets only)
65 - Entry time exceeded on platform ticket
66-69 Unknown/Unsued
70 - Near Field Radio error (Usually, two Oyster Cards presented at the same time, or other Oyster processing error), or other unspecified Oyster Card error
71 - Multiple cards (More than one Oyster card detected)
72 - Unknown/Unused
73 - Ticket given back for possible emergency use (e.g.. substitute bus service) and is not valid at this station
74-81 Unknown/Unused
82 - Illogical use of ticket
83 - Permit not valid at this time, used before time of validity
84-89 Unknown/Unused
90 - Gates set to reject this monitored ticket type (normally child-lock)
91 - Unknown/unused
92 - Card directory corrupt
93 - Card contents corrupt
94 - Card communications failed - Bad swipe of card/Card not read
95 - Unknown/unused
96 - Reader initialisation failed
96/98 - Unknown/unused
99 - UTS Gate Fault (Check Time-out)

These codes come into four catagories :

[1] #33 is when, say you exit through Monument station and re-enter at Tower Hill. The interchange is illogical, so won't accept your ticket.

[2] Pass is things like Staff/Dependant/Retired passes. Permits are things like Freedom Passes.

Both can be intercepted if reported lost or stolen (retained by machine, machine closes awaiting staff assistance. Code remains for staff to decipher)

The above codes also apply on National Railways barriers, which are virtually identical to the LU ones.