A seller can clear warning lights before a viewing. An OBD2 scanner reads the fault codes stored in the ECU — the ones they can't hide. Here is exactly how to use one at a viewing, what codes mean, and which scanners are worth carrying.
Every petrol and diesel vehicle sold in South Africa after approximately 2004 has an OBDII port — a standardised 16-pin connector usually located under the dashboard on the driver's side. The vehicle's ECU logs fault codes, sensor readings, and system status continuously. An OBD2 scanner reads this data.
When a warning light appears on the dashboard, a corresponding Diagnostic Trouble Code (DTC) is stored in the ECU. A seller can turn off the warning light by clearing the codes with a scanner — but the underlying fault remains, and the code will return once the engine runs through its monitors again. Some codes return within minutes of starting. More importantly, a scan run at the viewing reads everything stored right now, including codes set before the seller cleared them if they haven't driven far enough to set them again.
Clearing codes before a sale is common practice among dishonest private sellers and some dealers. Running a scan immediately when you arrive — before the engine has run long enough to reset monitors — is your best chance of catching recently-cleared codes. The "readiness monitors" status on the scan shows whether the ECU has completed its self-tests since the last code clear.
| Code | Meaning | Severity for buyers |
|---|---|---|
| P0420 / P0430 | Catalytic converter efficiency below threshold | Serious — R4,000–R15,000 to replace |
| P0300–P0308 | Random / cylinder-specific misfire | Serious — could be plugs, coils, injectors, or engine |
| P0171 / P0174 | System too lean (Bank 1 / Bank 2) | Investigate — vacuum leak, MAF sensor, injectors |
| P0401 | EGR flow insufficient | Moderate — common on diesels, R1,500–R5,000 |
| P0700 series | Transmission control system fault | Serious — gearbox faults, get specialist diagnosis |
| P0340 / P0345 | Camshaft position sensor fault | Moderate — R800–R2,500 but can cause no-start |
| B codes (Body) | Airbag, ABS, body control module | Safety-critical — never ignore airbag codes |
| U codes (Network) | Communication faults between modules | Investigate — can indicate damaged wiring or modules |
Any B-series airbag or SRS fault code is a walk-away trigger. An airbag system that has a stored fault may not deploy in an accident. Repair costs start at R8,000 and can reach R60,000+ if inflators, pretensioners, and the control module all need replacement.
A vehicle that has been driven normally for several weeks will show all or nearly all monitors as "Ready." A vehicle where codes were cleared recently will show multiple "Not Ready" monitors because the ECU has not completed its self-test cycles.
On petrol vehicles there are typically 8–11 monitors. On diesel vehicles fewer are applicable. If you see 3 or more "Not Ready" monitors on a vehicle that supposedly runs fine and has been driven to the viewing, ask the seller directly why the ECU has not completed its cycles. An honest answer is difficult to give if codes were recently cleared.
You do not need a workshop-grade scanner for a used car viewing. Three tiers cover most buyers:
A Bluetooth or WiFi ELM327 adapter plugs into the OBD port and sends data to your phone. The Torque Pro app (Android, ~R60 on Google Play) reads fault codes, live data, and readiness monitors on most vehicles. Adequate for checking codes at a viewing.
Limitation: generic ELM327 adapters do not read manufacturer-specific codes on BMW, Mercedes, or VW group vehicles well. Works best on Japanese and Korean vehicles.
A standalone scanner with a screen. The Launch CR3001 and Autel AL319 are well-regarded at this price point and available from Takealot and automotive retailers. Reads codes, live data, readiness monitors, and shows code definitions on the device — no phone or app needed at the viewing.
A full-system scanner reads not just engine codes but ABS, airbag (SRS), transmission, and body control modules. The basic OBDII standard only covers emissions-related engine codes. Airbag and ABS faults — which can be safety-critical — are only readable with a full-system scanner. If you are buying vehicles regularly or at higher price points, this is worth the investment.
Takealot stocks a reasonable range of scanners. For brands like Autel and Launch, also check AutoZone, Midas, and specialist suppliers like RS Components. Avoid the cheapest no-brand ELM327 adapters — counterfeit chips are common and connection quality is unreliable. Pay a little more for a recognisable brand.
An OBD2 scan is a powerful pre-purchase check but it has limits. It cannot detect mechanical wear — worn clutches, tired suspension, brake pad thickness, or engine bearing knock are not logged as fault codes. It cannot detect accident damage, rust, or paint repairs. It reads the vehicle's self-reported health, not its physical condition. Use it alongside the full 60-point inspection, not instead of it.
A basic Bluetooth OBD2 adapter and the Torque Pro app costs under R500 and fits in your pocket. It reads fault codes, checks for recently-cleared codes via readiness monitors, and gives you live engine data at any viewing. On any vehicle above R80,000, a full-system scanner is worth carrying. Neither replaces a physical inspection — but both give you information the seller cannot easily hide.
The complete inspection system for SA used car buyers. OBD scan plus physical checks — nothing missed.