Compression Tester: Buying guide
What it is
A compression tester screws into a spark-plug hole and measures the pressure each cylinder makes — a direct read on the mechanical health of the engine (rings, valves, head gasket).
Why you need it
When a misfire (P0300–P0304) will not clear after new plugs, coils and injectors, the cause is often mechanical. A compression test tells you whether a cylinder is low — pointing to worn rings, a burnt valve or a head-gasket problem — before you spend more on parts.
Features to look for
- A range that covers petrol engines (0–300 psi)
- A selection of thread adapters to fit different plug holes
- A flexible hose to reach recessed plugs
- A pressure-hold valve and an easy release button
- A clear, accurate gauge
How to choose
- A screw-in kit with several adapters fits more engines than a press-in tester.
- For diesels you need a dedicated high-pressure diesel compression kit instead.
- Look for a hold valve so you can read the peak after a few cranks.
- A "wet" test (a little oil in the cylinder) afterwards helps tell rings from valves.
Where to buy
We may earn a commission from purchases made through these links, at no extra cost to you.
Helps diagnose these codes
Related parts
Frequently asked questions
When should I do a compression test?
When a misfire code (e.g. P0302) stays after replacing the plug, coil and checking the injector — that points to a mechanical cause. A compression test shows whether that cylinder is low and why.
What do the numbers mean?
Most petrol engines read roughly 130–180 psi, and all cylinders should be within about 10% of each other. A single low cylinder suggests a valve, rings or a head-gasket issue; a wet test helps narrow it down.