Background and Identification
The ThinkPad T420 was introduced in 2011 as a mainstream business laptop. These machines are typically used in applications where a mobile workstation is not required and a standard laptop is sufficient.
You can identify your machine as a ThinkPad T420 or T420i by looking at the bottom right corner of the LCD bezel. The model of the machine is printed in this location.
To identify the factory specifications, look for a 7 digit System Type. It will look something like this:
- Ready to ship: XXXX-XXX
- CTO: XXXX-CTO
To find the specs on a machine with a CTO SKU, you will need to look up the system serial number and see what Lenovo lists under Machine Info in Lenovo Support. For ready to ship models, it can generally be found with a Google search or through Lenovo Support.
To locate the serial number, look on the same sticker that has your system SKU number. It should read like this: S/N XX-XXXXX. To locate the factory specs on a CTO model, you will need this to determine the exact specifications of the system, but it can also be used to determine if a ready to ship model has been customized with extra hardware or ordered with extra accessories when purchased, like extra batteries. It is also required to check the warranty on the system and will tell you when the system was purchased.
How the T420i varies from the T420
The T420i is similar in the sense it shares the same chassis, but the machine is less common.
While the T420i is very similar, these differences may matter to some buyers and need to be taken into consideration. For most buyers, these differences will be insignificant unless you are paying more for a T420i over a standard T420.
The T420i ships with the Core i3 2310M processor, which makes it somewhat unique. This processor was never offered on the standard T420. In addition, these systems lack AMT code presence in the BIOS.
While Intel AMT is not available, the Comutrace Option ROM is included with these systems.
The T420i can only be purchased with Intel HD Graphics. These systems were never offered with dedicated graphics.
This machine only accepts 8GB per Lenovo. However, the Core i3 2310M supports 16GB per Intel's ARK documentation.
The T420i uses an Intel 82577LC chipset. The standard T420 uses the 82577LM.
While the lack of AMT support is not always going to be acceptable, it provides a security benefit to security-conscious buyers who want to avoid having to disable it every time the BIOS is reset (but do not want to permanently disable it on a standard T420). There are known exploits in the version used by Lenovo that will likely never be patched.
These systems use a IGP T420 board with a different Ethernet chipset and do not have a discrete GPU option. These lack AMT BIOS code, but are otherwise identical. Due to these changes, the motherboard FRU is different on these altered boards but is otherwise identical besides firmware changes. This does not affect repair nor interchangeability of other parts on these systems.
- Intel® Core™ i3 2310M (2.1GHz, 3MB L3, 1333MHz FSB) Note: T420i only.
- Intel® Core™ i5-2410M (2.3GHz, 3MB L3, 1333MHz FSB)
- Intel® Core™ i5-2450M (2.5GHz, 3MB L3, 1333MHz FSB)
vPro (See Intel AMT)
- Intel® Core™ i5-2520M (2.5GHz, 3MB L3, 1333MHz FSB)
- Intel® Core™ i5-2540M (2.6GHz, 3MB L3, 1333MHz FSB)
- Intel® Core™ i7-2620M (2.7GHz, 4MB L3, 1333MHz FSB)
- Intel® Core™ i7-2640M (2.8GHz, 3MB L3, 1333MHz FSB)
Most machines use the Core i5 2520M. There are systems with i7 processors, but these are rare and are likely upgraded unless you can verify the system shipped with it by looking for a sticker with a Lenovo FRU or checking online.
Note: Lenovo did not ship these systems with quad-core processors. Systems with a QM processor are user upgraded. Do this at your own risk.
Most T420s ship with 4GB of RAM (unless upgraded by the owner or you have a CTO system). On a non-CTO SKU, you can quickly figure this out by googling the system type. For CTO systems, run the serial number through Lenovo's support page. Look at the Machine Identification section to see how to identify if you have a CTO or Ready to Ship model, and find your system serial number.
Memory capacity and type (Official)
- Memory type: DDR3 1.5V
- Memory Speed: 1333MHz (Recommended)
Maximum supported memory
- T420: 16GB
- T420i: 8GB
Memory upgrades (Unofficial)
Warning: Since the machine was written around the i5 2520M model, I cannot test 32GB compatibility.
- Supported Memory types: Sandy Bridge is DDR3L-Aware. While Lenovo does not support this, you should not have any problems with DDR3L modules installed. However, this may not be the case 100% of the time.
- DDR3L will run at 1.5V for compatibility, even though the machine is DDR3L-Aware.
- Memory speeds: 1600MHz (will downclock to 1333MHz)
Maximum memory capacity (Unofficial)
- T420 (With Core i7 upgrade): 32GB
- T420i (Core i3/i5 upgrade): 16GB
General information on DDR3L compatibility:
- While DDR3L will run stable in this system, it will most likely downgrade to 1.5V operation as Sandy Bridge is not 1.35V compatible.
- If dual voltage memory is installed and is accepted at 1.35V, you will see a marginal improvement in battery runtime.
- Do not use 1.35V only modules. These are not likely to work.
- To avoid any problems (if DDR3L is used), find modules that are advertised to downgrade to 1.5V to avoid compatibility problems.
- Do a 6-8 hour Memtest 86+ run and see if the system reports any problems running with DDR3L. If everything checks out, you are good to go.
This machine can run with 2 drives installed. It can take an mSATA SSD and a 2.5" SATA hard drive.
Note: While the WWAN slot can accept an mSATA SSD, the drive will only run at SATA II speeds in this slot.
While the WWAN slot only runs at SATA II speeds, this is sufficient for a small boot SSD. It is recommended that this is supplemented by a spinning hard drive for mass storage.
Important: The SATA spec on this machine is unclear. According to Crucial, this machine is SATA III capable. However, it has only been validated with SATA II drives. Since this is poorly documented, a SATA III drive with SATA II compatibility is recommended.
This machine shipped with 2 configuration options
- Intel HD Graphics 3000
- Intel HD Graphics 3000/nVidia NVS 4200M (nVidia Optimus enabled)
Important: When you are buying the system, decide if you want an Onboard video or Dual GPU system BEFORE you buy it. This cannot be upgraded.
This machine has a WiFi whitelist. In order to install other cards that are not listed, a no WL BIOS is required. These systems will POST with an 1802 error without this modification.
Supported (Not recommended)
- ThinkPad BGN (RealTek - Stockton) 1x1 BGN
- Intel Centrino Wireless-N 1000 (Condor Peak) 1x2 BGN
- Intel Centrino Advanced-N 6205 (Taylor Peak) 2x2 AGN
- Intel Centrino Ultimate-N 6300 (Puma Peak) 3x3 AGN
You must buy a card that has a Lenovo FRU on the label to ensure compatibility. Unless you flash a no WL BIOS or alter the PCI ID, you will need to make sure you have a Lenovo card. Shop for a card using the FRU you want to make sure it's going to work.
Note: All supported WWAN cards are obsolete, or rely on networks being phased out. Use an external device or flash a no WL BIOS and install a modern card.
The vast majority of machines are billed as WWAN upgradeable. This means the machine has the antennas and SIM card slot, but doesn’t include the WWAN card. The card is installed next to the lower RAM slot if it is not populated with an mSATA SSD.
Carrier activation block warning (READ BEFORE BUYING A LAPTOP WITH A WWAN CARD!)
WARNING: ALL WHITELISTED CARDS USE NETWORK TECHNOLOGIES THAT ARE BEING PHASED OUT, AND MOST CARRIERS WILL BLOCK SERVICE ACTIVATION. The information here is being left for archival purposes, or if you have an existing active card and can use it and when your final days of such end. You MUST flash a no WWAN WL BIOS to replace your card with something that supports 4G will be accepted as the WL in the BIOS blocks all other cards as they will not be present! 5G will not work on this laptop, as these WWAN cards are NGFF only.
At this point, the best option for cellular data on this laptop (and many similarly aged laptops) is to get an external hotspot, or use your phone if your plan includes hotspot data. DO NOT BUY A SPECIFIC T420 BECAUSE IT COMES WITH A WWAN CARD, EXPECTING IT TO BE ACTIVATED.
Ericsson F5521gw (3G/HSPA+/GSM capable)
- (No longer accepted on Verizon or AT&T due to 3G GSM network shutdown in 2022, and new activations blocked by Verizon). MNVOs will likely tell you no as well.
Sierra Wireless Gobi™ 3000 (HSPA/CDMA EV-DO) (No longer accepted on Verizon)
- No longer accepted on any US carrier due to 3G/CDMA shutdown - including many MNVOs.
IMPORTANT: READ BEFORE BUYING A USED WWAN CARD.
Dual network cards (GSM and CDMA):
These cards must be released from the previous account if used with a CDMA provider. This is a limitation of CDMA and is not a problem with GSM only cards. More specifically, this is a problem because CDMA relies on an embedded SIM card and the account is stored on the actual WWAN card.
Important: Even if you erase the eSIM, the card cannot be used as it is still associated with an account.
GSM only cards: If your WWAN card is GSM only, there is hope if it's attached to an account. GSM relies on a removable SIM, so these cards can typically keep being used. Check if the IMEI is clean before purchase.
While these cards are easy to reuse, there are two major downsides:
- The card may be blocked based on the IMEI
- If someone reports it stolen, the card will not work. This will likely be the end to mobile data on your ThinkPad since few US carriers are still activating 3G cards.
- Dual network cards with a residual account are useless. Check for this if the machine mentions WWAN. These CDMA cards typically use Verizon or Sprint.
- CDMA specific cards are on borrowed time! Eventually, all CDMA networks will be shutdown and these cards will be eWaste. Dual network cards are still safe (for now).
- GSM cards are safer, and can typically be used on multiple accounts. These can be blocked with the IMEI as well.
Known issues with used systems
Computrace Persistence Module (And how to check)
These machines support the Computrace Persistence Module. This can be a problem if the subscription was left on the machine itself before sale. It is important to check this before buying the machine. To do this, do the following:
- Boot up the machine, and get to the BIOS. You can do this in a few ways, but the easiest way to do it is to press F12 and load the BIOS from this menu. This is also a good time to check for an SVP, POP or HDP.
- If the machine warns you that Computrace is active and it cannot be disabled, the subscription is active. DO NOT use the machine in this state.
If you see this when the BIOS loads, the machine has an active subscription left on it:
To remove this, you will need to contact Absolute Software and request the subscription be removed. If everything is good, they will remove it.
Note: If you use a non-Microsoft OS, you will need to install Windows temporarily to unlock Computrace.
Once it is released, Computrace should be disabled to avoid problems later on. This can be soft disabled (Recommended) or permanently disabled (Not recommended). It may also be a good idea to save the receipt or get notarized transfer papers in case proof is needed.
There are 3 BIOS password levels that apply to this system.
Supervisor password (SVP)
Lenovo systems do not provide a backdoor password. If this password is lost, you will need to replace the motherboard. While this password can be removed without a new motherboard, these methods will not be discussed on this device page.
Hard Disk password (HDP)
Note: This is not an issue if you know the master password for the system. The master password can be used to unlock the hard drive.
These passwords cannot be removed without replacing system hardware, generally speaking. Some systems back this up to the ATMEL chip, so this password may be copied to your replacement hard drive. This will render the new hard drive useless.
If you are not sure if this will happen, install a hard drive you do not mind losing for verification. If it doesn't copy over, it is safe to install the new hard drive. If it does, you will need to reset the motherboard before replacing the hard drive.
Power on Password (POP)
Note: Before resetting this, check for an SVP. Doing this to a system with a POP and SVP will cause the system to default to the SVP and render the system useless!
Unlike other passwords, this is easy to reset. To remove a POP from the system, remove the CMOS battery and hold the power button for 30 seconds to ensure the CMOS RAM has no reserve power. Upon rebooting, you will run into a checksum error where the machine will use the factory default BIOS settings.