OBU guide
How to check your OBU card balance
Where the balance actually lives, the ways to see it — with or without the OBU’s touchscreen — and what every card status the unit can report actually means.
Last reviewed September 2026
Where the balance actually lives
Your stored-value card sits in the OBU’s processing unit — the box in the right-hand side of the front passenger footwell, not the screen and not the antenna. That is why the balance is something the OBU reports rather than something your phone works out: the card is physically in the car, and only the OBU can read it.
It also explains a common surprise. If you never installed the touchscreen, nothing in your car is showing you that number by default — the card is still there and still being charged, you just have no window onto it. That’s covered in no OBU touchscreen? what you can still see.
Front-end vs back-end payment
Before you go hunting for a missing balance, check which way your OBU is paying. There are two modes, and they behave completely differently:
- Front-end payment deducts from the stored-value card in the processing unit at the moment of the charge. This is the mode where a balance exists, matters, and can run out mid-drive.
- Back-end payment settles the charge to an account afterwards instead of taking it from a card in the car. In this mode there may be no card in the unit at all — and a status of “no card” is then completely normal rather than a fault.
The OBU reports which mode it is in, so if you are seeing a “no card” state and everything else is working, the payment mode is the first thing to check rather than the card.
Three ways to check it
- The OBU’s touchscreen, if you had one installed. Showing the card balance is one of the things it is there for.
- A compatible mobile app. When LTA made the touchscreen optional, the stated alternative was that drivers get the key information — charges and balance included — through a compatible mobile application. Your phone has to be registered with the OBU first; that is a one-time job at ERP Online Services, and we walk through it in register your phone with your OBU.
- A dedicated in-car display. A phone in your pocket is not a glanceable balance while you drive. Gantry Display is a round AMOLED screen that keeps the balance in view, and blinks it when it drops below a threshold you set — because the moment you want to know is before the deduction fails, not after.
Every card status, explained
The OBU doesn’t just report a number — it reports a state for the card, and the state is what tells you whether a top-up will fix things. These are all of them, in plain English:
| Status | What it means for you |
|---|---|
| Detected | The normal, healthy state — a card is in the unit and readable. This is what you should see. |
| Insufficient Stored Value | The card works, but there isn’t enough on it for the charge. This is the common one, and it is a top-up away from fixed. |
| Expired Stored Value | The card is past its expiry date. Stored-value cards do expire — check the date printed on it. |
| Blacklisted | The card has been blacklisted by its issuer and won’t be accepted. You need to take it up with the issuer. |
| Blocked | The card is blocked. As with blacklisting, this is between you and the card issuer. |
| Invalid | The unit can read something, but it isn’t a card it accepts. |
| Issuer ID Error | The card’s issuer identifier isn’t one the unit recognises. |
| Faulty Stored Value | The card is being read but its stored value is faulty — usually a damaged or failing card. |
| Wrong Stored Value Debit Certificate | A security check on the deduction failed. Rare, and one for LTA or the issuer rather than something you can fix in the car. |
| NFC Access Error | The unit couldn’t complete the contactless read. |
| No Card (front-end payment) | No card is inserted, or the unit is set to a payment mode that doesn’t use one. See back-end payment below before assuming something is wrong. |
Only the first two are everyday states. Everything below them is a card problem rather than a road-pricing problem, and most of them are resolved with the card issuer or through LTA’s own channels — not from inside any app.
When a deduction fails
A charge runs as a short sequence: an alert as you approach the charging point, a pre-charging message, then either deduction successful or deduction failure. A failure is the OBU telling you the charge could not be settled from the card — overwhelmingly because of insufficient value, occasionally because of one of the card states above.
Unpaid ERP charges don’t simply evaporate, so the useful habit is to notice the balance falling rather than to discover the failure afterwards. Whatever you use to watch it, watch it before the trip that matters.
One thing no app can do. The interface LTA publishes for third-party apps is read-only. Gantry — or any other app — can show you the balance, the status, and every deduction as it happens, but cannot enable or disable a card, top it up, or change how the OBU pays. Those stay with the OBU itself and LTA’s own channels.