obd-diagnosis

name	obd-diagnosis
description	Interpret OBD-II diagnostic trouble codes and provide diagnostic guidance

OBD Diagnosis Skill

Use this skill when the customer provides OBD-II diagnostic trouble codes.

Code Structure

Code Format: XNNNN

• First character (type):

  • P = Powertrain (engine, transmission)
  • B = Body (HVAC, airbags, lighting)
  • C = Chassis (ABS, stability control)
  • U = Network (communication between modules)

• Second character (origin):

  • 0 = Generic (SAE standard)
  • 1 = Manufacturer-specific
  • 2-3 = Varies by manufacturer

• Third character (subsystem):

  • For P codes:
    • 1, 2 = Fuel and air metering
    • 3 = Ignition system
    • 4 = Emissions controls
    • 5 = Speed/idle control
    • 6 = Computer systems
    • 7, 8 = Transmission

Diagnostic Approach

1. Look Up the Code

Use lookupObdCode tool for common codes.

2. Understand Context

Ask about:

• When did the light come on?
• Any performance changes?
• Recent repairs or maintenance?
• Has this code appeared before?

3. Identify Root Cause vs Symptom

Many codes are symptoms of underlying issues:

• P0300 (random misfire) could be caused by ignition, fuel, or mechanical issues
• P0420 (catalyst efficiency) is often caused by upstream problems
• Multiple codes may share a root cause

4. Consider Related Codes

Codes often come in clusters:

• P0171 + P0174 = System lean on both banks → vacuum leak or MAF issue
• P0301-P0304 = Misfires on specific cylinders → individual cylinder issues
• Multiple U codes = Communication problem affecting many modules

Severity Classification

Critical (Stop Driving)

• Misfire codes with flashing CEL
• Oil pressure codes
• Overheating codes
• Brake system codes

High (Service Soon)

• Catalyst efficiency (P0420/P0430)
• Transmission codes
• Fuel system lean/rich
• Ignition system codes

Medium (Schedule Service)

• EVAP system codes
• Sensor circuit codes
• Secondary air codes
• EGR codes

Low (Monitor)

• Occasional pending codes
• Minor sensor codes
• After clearing, doesn't return

Common Code Families

Misfire Codes (P0300-P0312)

• Check spark plugs, coils, injectors
• Compression test if persistent
• May indicate head gasket if coolant loss

Oxygen Sensor Codes (P0130-P0167)

• Upstream sensors affect fuel trim
• Downstream sensors monitor catalyst
• Often caused by exhaust leaks first

EVAP Codes (P0440-P0457)

• Often just a loose gas cap
• Check filler neck and hoses
• Can indicate purge valve issues

Catalyst Codes (P0420-P0439)

• Usually indicates cat failure
• But check upstream issues first
• Exhaust leaks can cause false codes

Response Format

When interpreting codes, provide:

1. Code Breakdown: What each code means
2. Likely Causes: Most common reasons (in order of likelihood)
3. Severity Rating: Critical / High / Medium / Low
4. DIY Checks: What the customer can verify
5. Professional Diagnosis: What a shop would check
6. Related Symptoms: What else to look for

Example Response

"You've provided code P0171 - System Too Lean (Bank 1).

What This Means: The engine is receiving more air than fuel on one side of the engine. The computer is trying to compensate by adding fuel but has reached its limit.

Likely Causes (most to least common):

1. Vacuum leak (intake manifold gasket, vacuum hoses)
2. Faulty Mass Air Flow (MAF) sensor
3. Fuel delivery issue (weak pump, clogged filter, failing injector)
4. Exhaust leak before O2 sensor

Severity: Medium - Safe to drive short-term but fuel economy and performance suffer. Can cause catalyst damage if ignored.

DIY Checks:

• Listen for hissing sounds under the hood
• Check for cracked or disconnected vacuum hoses
• Clean the MAF sensor with MAF-specific cleaner

Professional Diagnosis: Smoke test for vacuum leaks, fuel pressure test, live data review.

Do you have any other codes? And have you noticed any performance issues like rough idle or hesitation?"