# Engine Diagnostic Tests Explained, Upland > What each engine diagnostic test proves: scan data, smoke, fuel pressure, compression, leak-down and oil pressure. Fee stated first in Upland. Source: https://route66autorepairshop.com/engine-services/engine-diagnostic-test Updated: 2026-09-17 Business: Route 66 Auto Repair, 1775 W Foothill Blvd, Upland, CA 91786, (909) 608-7699 Hours: Monday to Friday 8 AM–5 PM, Saturday 8 AM–2 PM, Sunday closed **TLDR:** Each test answers one question. A **leak-down test** finds where compression escapes, a **smoke test** finds vacuum leaks, and the **fee is stated first**. An engine diagnostic test at Route 66 Auto Repair in Upland is not one machine that prints an answer. It is a set of specific tests, each proving or ruling out one cause. This page lists the tests our ASE-certified technicians use, what each one tells us, and why we pick some and skip others. The counter states the diagnostic fee before any test begins. ## Which tests do we use, and what does each prove? - Scan with freeze frame: which fault set, and at what speed, load and temperature - Live data review: whether sensors agree with each other and with the engine's real condition - Smoke test: pushes visible vapor into the intake or evaporative system to reveal leaks - Fuel pressure and volume test: whether the pump and regulator deliver what the engine asks for - Compression test: how well each cylinder holds pressure while cranking - Leak-down test: where that pressure escapes, past valves, rings or a head gasket - Oil pressure test with a mechanical gauge: whether the dash sender is telling the truth ## Compression test or leak-down test: what is the difference? A compression test cranks the engine with a gauge in each spark plug hole and compares the numbers. It tells us that a cylinder is weak, but not why. A leak-down test fills a cylinder with shop air at the top of its stroke and measures how much escapes. Then we listen. Air hissing from the tailpipe points to an exhaust valve. From the throttle body, an intake valve. From the oil filler, the rings. Bubbles in the coolant point to a head gasket. Together they turn a vague weak cylinder into a named cause, which is the difference between a [cylinder head repair](https://route66autorepairshop.com/engine-services/cylinder-head-repair) and a [piston ring replacement](https://route66autorepairshop.com/engine-services/piston-ring-replacement). ## How we choose the tests 1. **Start broad:** Every visit begins with a scan and live data, because they point to the next test. 2. **Explain before testing:** The service writer tells you which tests are planned and why. 3. **Test the suspect:** Only the tests the data calls for are run, in order of speed and certainty. 4. **Record the numbers:** Readings go on your repair order so you can compare them later. ## What's included in an engine diagnostic test - Scan with freeze frame review - Live data comparison - The pinpoint tests explained at intake - Test readings written on the repair order - Written estimate for any repair found ## What test results mean for urgency **Need now:** - Very low oil pressure on a mechanical gauge - Combustion gas in the coolant with overheating **Need later:** - One cylinder slightly lower than the rest with no misfire - A small evaporative leak found on a smoke test ## How pricing and timing work The diagnostic fee is stated at the counter before testing and applies to approved related repair. Compression and leak-down tests take longer on engines where the spark plugs sit under the intake manifold. We tell you that at intake. For a broader look at drivability, see our [engine diagnostics](https://route66autorepairshop.com/engine-services/engine-diagnostics) page. ## Aftercare Keep your readings. If you monitor an older engine, a second compression test a year later shows whether wear is steady or speeding up. Tests are also part of a [pre-purchase inspection](https://route66autorepairshop.com/diagnostics-and-inspections/pre-purchase-vehicle-inspection) when a buyer wants engine health in writing. ## Frequently asked questions ### Is a code scan the same as an engine diagnostic test? No. A scan is the first step. It shows which fault the computer saw. The tests that follow show which part or condition caused it. ### Will a compression test hurt my engine? No. It uses the starter to crank the engine with the fuel and ignition disabled. Spark plugs come out and go back in with the correct torque. ### Can you test an engine that will not start? Yes. No-start tests focus on spark, fuel pressure, compression and crank sensor signals. We accept tow-ins for cars that cannot be driven. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.