Detailed Diagnostic
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An in-depth diagnosis for trickier faults.
How it works
- Submit your fault codes, symptoms, and any relevant history.
- A professional mechanic reviews everything personally.
- You get a full fault-finding plan, with diagrams to support mechanical and wiring checks.
How it works
From your details to your answer.
You send the details
Add your fault codes, your vehicle and what it's doing when you book. Takes a minute.
A real mechanic reviews it
We work through your codes against your symptoms. Never an automated report.
Your diagnosis arrives
You get the likely cause and clear next steps by email. See a real example below.
Example
What a Detailed Diagnostic looks like.
A real example for a 2014 Ford Focus 1.6 with codes P0301 and P0171. Yours will be a full plan written for your car.
Thanks for the full details. Codes P0301 (cylinder 1 misfire) and P0171 (system too lean, bank 1), with a cold rough idle that appeared after a motorway run.
HOW THESE TWO CODES RELATE
A single cylinder misfire plus a bank wide lean code usually means one of two things: an ignition fault on cylinder 1 that the ECU is compensating for, or an unmetered air leak that upsets cylinder 1 most at idle. The plan below separates the two.
STEP BY STEP PLAN
- Ignition side, most likely. Swap the cylinder 1 coil to cylinder 2 and swap the plug too. Clear codes, then drive a cold start and a motorway cycle. If the misfire moves to cylinder 2, the coil is proven, so fit a new one.
- If the misfire stays on cylinder 1. Measure cylinder 1 compression (expect it within about 10% of the others). Low compression points at a valve or ring issue, not electrical.
- Fuel and air side, for the lean code. At idle, check short term fuel trim. High at idle but normal at higher RPM strongly suggests a vacuum or intake leak. Inspect the intake boot, PCV hose and brake servo hose, and smoke test if you can.
- Airflow. Clean the MAF sensor and check the air filter is not restricting flow.
- Re-scan after each step so you only change one variable at a time.
MOST LIKELY OUTCOME
A failed cylinder 1 ignition coil, with the lean code clearing once the misfire is fixed. Second most likely is a small intake or vacuum leak.
Parts to have ready if you want to fix it in one go: one ignition coil and one spark plug for cylinder 1. Hold off on anything else until the trims and compression are checked.
Reply with your fuel trim readings or anything you find and we'll refine the plan.
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