On-Site Diagnosis: the full procedure
Run a structured, documented diagnosis so the root cause — not just the symptom — is identified before any parts are quoted.
- Applies to: ASE-certified technician
- Frequency: Every diagnostic or repair job
- Scope: Covers the diagnostic workflow and documentation. The actual technical interpretation and any test requiring lifting/refrigerant defer to OEM service information, ASE-certified judgment, and (for A/C) EPA Section 609.
What you need
- OBD-II scan tool
- Multimeter
- OEM service/diagnostic info
- Test lights/pressure gauges
- Inspection light
- Customer’s symptom notes
The procedure, step by step
- Review the intake — Re-read the captured symptom and verify it with the customer; reproduce the complaint if safely possible.
- Pull codes and freeze-frame — Connect the scan tool, record all stored/pending DTCs and freeze-frame data before clearing anything.
- Follow OEM diagnostic path — Use the OEM/Mitchell/AllData diagnostic tree for the codes and symptom rather than guessing; let ASE-certified judgment drive the test sequence.
- Test and measure — Perform the indicated tests (electrical, mechanical, pressure) and record actual readings against spec.
- Isolate root cause — Distinguish the failed component from secondary effects; confirm with a verifying test before concluding.
- Defer regulated tests — Any A/C performance test that involves refrigerant defers to a Section 609-certified tech with certified equipment; any test requiring lifting follows the safe-work-area SOP.
- Document the finding — Record codes, readings, root cause, and the recommended repair with parts and labor needed.
- Brief the customer — Explain the finding in plain language and set up the written estimate before any repair begins.
Quality check before you finish
- Customer complaint verified/reproduced where safe.
- All DTCs and freeze-frame data recorded before clearing.
- OEM diagnostic procedure followed for the codes/symptom.
- Actual test readings recorded and compared to spec.
- Root cause confirmed with a verifying test, not assumed.
- A/C/lifting-dependent tests deferred to the proper certification/SOP.
- Finding documented and explained to the customer before quoting.
This is a free, source-anchored standard operating procedure (SOP) you can print and hand to staff. It documents the work sequence for a Mobile Mechanic business — not safety or regulatory rulings, which defer to the cited authorities, the applicable code, and your own health-and-safety plan. Open the tool above to print it, toggle ink-saver, or (with a free ToolFluency Business account) edit it to match your own workflow.
Sources
- OEM service information (alldata.com)
- ASE (ase.com)
- EPA Section 609 (epa.gov)
About Free On-Site Diagnosis SOP
Free printable SOP for mobile mechanic diagnosis: pull codes, follow the OEM diagnostic path, confirm root cause, and document before quoting.
How to use
- Read the full procedure top to bottom before the work — the SOP runs in order and each step builds on the last.
- Toggle Ink-saver (black & white) for a cheaper mono print for the binder; leave it off for the full-color version.
- Click Print SOP to print or save as PDF. Print one per crew, laminate it for the binder, or attach it to the job in your scheduling system.
- Train new hires on it and have staff sign off. Found something out of date? Use the feedback link — flagged SOPs are re-researched against the source list.
Frequently asked questions
Why record DTCs and freeze-frame before clearing codes?
Freeze-frame captures the conditions when the fault set and is often the key to root cause. Following the OEM diagnostic tree for those codes — an ASE-certified technician’s task — prevents guess-and-replace parts cannons.
How is an A/C diagnosis handled?
Any A/C test that involves accessing or measuring refrigerant defers to a tech certified under EPA Section 609 using EPA-certified equipment. The SOP governs the documentation and customer briefing around it, not the refrigerant procedure itself.
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