Jeep 4.0 Crank Position Sensor: Symptoms, the Hot-Start Test, and the Fix

Few failures on the 4.0L inline-six are as common — or as misdiagnosed — as the crankshaft position sensor, usually shortened to CPS or CKP. It's the sensor that tells the PCM where the crankshaft is and how fast it's turning. Without that signal, the computer won't fire the injectors or the ignition coil, so the engine cranks and cranks but never catches. On XJ Cherokees (1991–2001) and TJ Wranglers (1997–2006), a dead or dying CPS is one of the single most frequent causes of a stranding no-start.

Here's the one-line version: if your 4.0 starts and runs cold, then stalls and won't restart once it's at operating temperature — only to fire right up after it cools off — replace the crank position sensor before you touch anything else. That heat-soak pattern is the CPS's signature, and almost nothing else on the engine behaves that way.

What are the symptoms of a bad crank position sensor?

CPS failures show up in a few recognizable ways, and they tend to get worse before they leave you stranded for good:

Thermal no-start. The headline symptom. Cold, the Jeep runs perfectly. After it warms up — or after a hot restart at a gas station — it cranks but won't fire. Let it sit 20 to 30 minutes and it starts like nothing happened. Heat opens the failing circuit; cooling closes it again.

Random stalling. The engine dies at idle or low speed for no clear reason, sometimes catching itself, sometimes not. It may stumble or surge just before it quits.

Crank, no spark. When you check, there's no spark at the plugs and the injectors aren't pulsing — because the PCM never saw the crank signal it needs to fire them.

"No Bus" on the odometer, or no code at all. A flickering or "No Bus" message can accompany the no-start. Just as often the check engine light never comes on, because an intermittent signal that recovers doesn't always set a code. When a code does store, it's usually P0320 — no crank reference signal at the PCM.

Where is the crank position sensor on a 4.0?

The CPS lives at the top of the bellhousing on the driver's side, where the transmission bolts to the back of the engine. It's tucked low and rearward, reading the teeth of the flexplate (automatic) or flywheel (manual) through a slot. A single bolt holds it, and its harness connector clips into the engine harness and routes forward along the block. It's a 4.0L Hall-effect, three-wire sensor: a 5-volt reference, a ground, and a signal wire that switches between roughly 5 volts and zero as the teeth pass.

How do I test it — or should I just replace it?

Be honest about what testing can and can't do here. Because the CPS is a Hall-effect sensor, you cannot bench-test it with a simple resistance check — an ohmmeter tells you nothing about whether it's producing a usable signal. That's why so many people "test good" and stay stranded.

With a scan tool. The cleanest check: watch engine RPM on a scanner while a helper cranks. If RPM reads 0 while the starter is spinning the engine, the PCM is getting no crank signal — sensor or its wiring. This is the most direct confirmation.

With a multimeter. Back-probe the connector with the sensor plugged in. Confirm a steady 5-volt reference and a good ground first — if either is missing, you have a wiring or PCM problem, not a sensor. Then watch the signal wire in DC volts while cranking; a healthy sensor pulses between about 5 volts and 0 as the flexplate teeth pass.

When to stop testing and replace. The cruel part of an intermittent CPS is that it usually passes every test while it's cold — the exact time you can reach it. If your symptoms match the thermal no-start pattern, the reference and ground check out, and there's no spark when it's hot, the sensor is the answer. A new quality CPS is cheap insurance against chasing a ghost for weeks.

Which sensor to buy, and why the cheap one bites back

This is the part where saving ten dollars costs you the whole repair twice. The 4.0L PCM is unusually sensitive to the strength of the signal the sensor sends. The cheapest no-name sensors use weaker magnets and lower-grade internals, and the computer can't reliably read them — so the brand-new bargain part reproduces the exact intermittent no-start you were trying to cure. It's one of the most common repeat-failure complaints on every Jeep forum.

Buy genuine Mopar or a known brand. A factory Mopar sensor or a quality unit from Crown or Standard (SMP) is worth the premium. Expect roughly $40–$110 for a good one; the $10–$25 specials are exactly the parts people warn each other about.

Match it to your year. The connector and sensor changed across the run, so verify against your VIN. For the late XJ/TJ 4.0 the common Mopar number is 56027866AE (2000–2001), with 56027866AC covering 1999; earlier years use different part numbers. Confirm before you order — the wrong one looks close but won't seat or signal right.

The fix

Replacement is a roughly 30–60 minute job and needs no special tools. Disconnect the battery, trace the harness from the top of the bellhousing and unplug the connector, then remove the single retaining bolt and pull the old sensor out of its slot. Seat the new one fully, torque the bolt snug, and route the new harness exactly where the old one ran so it stays clear of the exhaust and the starter. The tight access from above is the only real annoyance; a long extension and a swivel earn their keep here. Reconnect the battery, and the thermal no-start that's been ruining your week should be gone.

One honest caveat: a no-spark, no-start can occasionally trace to the ignition coil, the ASD relay, or wiring rather than the sensor. But when the failure is heat-dependent — dead hot, alive cold — the crank position sensor is the overwhelming favorite, and it's the cheapest, most likely fix to rule in first. Start there, buy the good one, and you'll most likely end the no-start for good.