You plug in a scanner, it spits out P0302, and suddenly you are on YouTube watching a video titled "YOUR ENGINE IS ABOUT TO BLOW."
Most of the time, it is not.
DTCs are useful if you know how to read them. Useless — or actively misleading — if you do not. This guide covers how the 720S organizes codes, what the different statuses mean, and a reference table of the codes I actually encounter.
OBD2 codes follow a pattern:
The first digit after the letter tells you if it is a generic code (0 — every manufacturer uses it the same way) or manufacturer-specific (1 and up — McLaren defines what it means).
Example: P0302 is a generic code for "Cylinder 2 misfire." B1234 would be McLaren-specific and you need the SIS to decode it.
This is where most people panic unnecessarily.
Pending: The module detected a fault once but has not seen it enough times to confirm it. Could be a glitch. Could be the start of a real problem. Clear the code, drive the car for a few days, and see if it comes back.
Confirmed (Stored): The module has seen the fault multiple times across multiple drive cycles. This is a real problem that needs attention.
Permanent: Cannot be cleared without fixing the underlying issue. These are serious — pay attention to these.
I have seen P0171 (system too lean) come up as pending after a bad gas fill, then never return. I have also seen it come up as pending, get ignored for six months, and turn into a confirmed code with an oxygen sensor destroyed by running rich. The difference is paying attention to whether it returns.
P0300-P0306: Misfire codes. P0300 is random/multiple cylinders. P0301 through P0306 point to specific cylinders (1-6). First thing to check: spark plugs and ignition coils. The M840T runs hot and plug life is shorter than McLaren suggests.
P0171/P0172: System too lean/rich on bank 1. Usually a vacuum leak, dirty MAF sensor, or fuel pressure issue. Check for cracked intake boots first — they dry out and crack over time.
C0035-C0040: Wheel speed sensor circuits. These show up after car washes more than anything else. Water gets into the connector, causes an intermittent fault, sets the code. Clean the connector, clear the code, move on.
P0420: Catalyst efficiency below threshold. On a 720S with low mileage, this is usually a bad oxygen sensor, not a dead catalytic converter. Confirm before spending ,000 on an exhaust system.
U0100-U0199: CAN communication losses. These mean one module stopped talking to another. Usually a wiring issue or a module that needs resetting. Check battery voltage first — low voltage causes phantom communication errors.
A 0 OBD2 scanner reads generic P-codes. MDS reads everything: manufacturer-specific codes, module health status, live data streams, adaptation values, and service mode functions.
If you are serious about maintaining a 720S, MDS is worth the investment. The cheap scanners will miss half of what is wrong with your car.
Honest answer: if the Check Engine Light is flashing (not steady), pull over. A flashing CEL means a severe misfire that is dumping unburned fuel into the exhaust and can destroy the catalytic converter in minutes.
Steady light? You have time. Read the code, look it up, plan the repair. Do not replace parts based on a code alone — codes tell you what is wrong, not necessarily why. A misfire code could be a spark plug, a coil, a fuel injector, a vacuum leak, or a timing issue. Diagnose before you buy.
More guides: common problems, maintenance schedule, fluid specs.