Why a scan alone is not a diagnosis
Plugging in a code reader and reading P0420 tells you the catalyst efficiency is below threshold. It does not tell you whether the cat is failing, an oxygen sensor is lazy, there is an exhaust leak before the rear sensor or a misfire is overloading the cat. Every code has a list of possible causes and a workshop that just sells you the part listed first is gambling with your money.
Our diagnostic process
We start with a full system scan across engine, transmission, ABS, airbag and body modules to see if related codes paint a clearer picture. We then read freeze frame data, live data and any pending codes. From there we test the actual circuit, sensor or component using a multimeter, scope or smoke machine. We do not start replacing parts until we have isolated the fault.
Common faults we see
Misfires from worn coils or plugs, lean codes from intake leaks or failing MAF sensors, EGR and DPF issues on diesels, knock sensor codes on Subarus, VVT solenoid faults on Toyotas, mechatronic codes on Volkswagen DSG transmissions and limp mode caused by turbo overboost on common rail diesels. We see these every week and know where to look first.
Scan tools and software
We run Autel, Launch and brand specific tools where required so we can access modules and bidirectional functions that generic OBD2 readers cannot. This matters most on European and Japanese vehicles where many tests require a tool that can command the ECU, not just read from it.
What you get
A written report with the codes found, the tests performed, the cause identified and a quote for the repair. If multiple options exist we explain the trade off. If the fix is simple we often do it in the same visit. Diagnostic time is quoted up front before we start.