SYMPTOM-TO-SYSTEM DECODER
Customer says "my car does X." Your job is to translate that into "this system is likely the cause, here are the next 3 tests." This decoder maps the most common customer complaints to root systems + the basic checks. Where deeper diagnostic is needed, route to A1-A9 modules.
SYMPTOM DECISION TREE
Customer complaint at the top → system + first test at the bottom. Use this flow to route the right diagnostic before touching anything.
ENGINE COMPLAINTS
| Customer says | System | First checks |
|---|---|---|
| "Won\'t start" | Starting / fuel / ignition | Battery test, fuel pressure, scan for codes (route → A6, A8) |
| "Cranks slow" | Starting / battery | Battery load test, starter draw (route → A6) |
| "Runs rough at idle" | Engine performance | Vacuum leak, MAF, ignition, fuel pressure (route → A8) |
| "Stalls when stopping" | Fuel / idle control | IAC, vacuum leak, low fuel pressure (route → A8) |
| "Loses power going up hills" | Fuel / boost / ignition | Fuel pressure under load, plugs, MAF (route → A8) |
| "Check engine light on" | Engine management | Pull codes first; route to area-specific module |
| "Smells gasoline" | Fuel system | Fuel line/injector leak, EVAP fault (route → A8) |
| "Smokes black" | Fuel / air ratio | Lean/rich condition (route → A8 or A9) |
| "Smokes white" | Coolant in chamber OR cold diesel | Head gasket check, coolant level, EGT (route → A1 or A9) |
| "Smokes blue" | Oil burning | Valve seals, rings, PCV (route → A1) |
| "Overheats" | Cooling system | Coolant level, thermostat, fan, water pump (route → A1) |
| "Misfire feeling under acceleration" | Ignition / fuel | Coil, plug, injector (route → A6, A8) |
DRIVELINE / TRANSMISSION COMPLAINTS
| Customer says | System | First checks |
|---|---|---|
| "Slips shifting" | Auto transmission | ATF level + condition, scan codes (route → A2) |
| "Won\'t shift into gear" | Transmission / shifter linkage | Shift cable, range sensor (route → A2) |
| "Whine that changes with speed" | Differential / bearing | Diff fluid, wheel bearing test (route → A3) |
| "Clunk when shifting from P to D" | U-joint / CV / motor mount | Visual inspection, mount check (route → A3) |
| "Vibration at highway speed" | Tire balance / driveshaft | Balance, runout, U-joints (route → A3, A4) |
| "Pulls hard to one side" | Steering / brake drag | Tire wear, alignment, brake caliper free play (route → A4, A5) |
BRAKE COMPLAINTS
| Customer says | System | First checks |
|---|---|---|
| "Squeals when stopping" | Brake pads | Pad wear indicator contact, glazed pads (route → A5) |
| "Grinds when stopping" | Pads worn through | Pad measurement + rotor inspection (route → A5) |
| "Vibrates when braking" | Rotor warp | Rotor thickness variation, measure (route → A5) |
| "Pulls to one side when braking" | Caliper / hose | Caliper drag, hose restriction (route → A5) |
| "Pedal goes to the floor" | Hydraulic / leak | Master cylinder, hose leak, fluid loss (route → A5) |
| "ABS light on" | ABS / wheel speed sensor | Scan codes, sensor inspection (route → A5) |
SUSPENSION / STEERING COMPLAINTS
| Customer says | System | First checks |
|---|---|---|
| "Wanders on the highway" | Steering / alignment | Tie rod play, alignment, tire pressure (route → A4) |
| "Bouncy / floats" | Shocks / struts | Bounce test, visual leak (route → A4) |
| "Clunks over bumps" | Suspension component | End link, ball joint, control arm bushing (route → A4) |
| "Tires wear unevenly" | Alignment / wear pattern | Toe, camber, caster, pressure (route → A4) |
| "Steering wheel off-center" | Alignment | Toe adjustment, alignment angles (route → A4) |
| "Heavy steering" | Power steering | PSF level, belt, pump (route → A4) |
ELECTRICAL / HVAC COMPLAINTS
| Customer says | System | First checks |
|---|---|---|
| "Battery keeps dying" | Charging / parasitic draw | Battery test, alternator test, parasitic draw test (route → A6) |
| "Lights dim at idle" | Charging | Alternator output, belt tension (route → A6) |
| "Headlight burned out" | Lighting | Bulb test, fuse check (route → A6) |
| "A/C blows warm" | HVAC refrigerant / compressor | Pressure check, clutch operation (route → A7) |
| "No heat" | HVAC / coolant | Coolant level, blend door, thermostat (route → A7) |
| "A/C smells musty" | HVAC cabin filter / evap | Cabin filter, evaporator clean (route → A7) |
| "Wipers don\'t work" | Wiper motor / linkage / fuse | Fuse, switch test, motor (route → A6) |
VERIFY THE COMPLAINT BEFORE YOU FIX
Customer says "the brakes squeal" but the noise is actually a wheel bearing. Or "transmission slips" but it\'s a misfire under load. Always road test to recreate the symptom before you start swapping parts. If you can\'t recreate it, ask the customer to ride along — most "intermittent" issues happen at specific speeds, temperatures, or driving conditions.
Document what you reproduced: "Brake squeal recreated on left front at 35 mph low-speed stops, consistent with pad wear indicator." That gives the advisor + customer confidence + protects you if the customer claims you fixed the wrong thing.