Code reader will not connect to system?

BLAZ85

MEMBER
2002 FORD F-150
5.4L
V8
4WD
AUTOMATIC
190,000 MILES

This a vehicle belonging to a friend.
When the code reader is plugged into the OBD port it will not connect to the system to read the codes. The check-engine light is "on". I suspect it lacks either power or ground.

A little history; a few weeks ago, a shop replaced the intake manifold. Vehicle ran fine after the replacement (no CEL's). It is my understanding that a large portion of the wiring harness under the hood must be removed for manifold replacement. This may be a contributor to the problem.

Two days ago vehicle developed a misfire and CEL appeared (suspect defective coil). When we tried to read codes, we could not. In the past there was no problem

Could you provide me with the following: Fuse number and location that provides power to port, Pinout of port in particular power and ground pins, wiring diagram of power and ground on the port and any other insight on correcting the problem.

Thank you in advance for your response!

Friday, May 16th, 2025 AT 9:55 PM

3 Replies

STEVE W.

MECHANIC
14,726 POSTS

This is the basic layout of an OBD port. Pin 16 is battery power from fuse 3 in the central junction box (under left dash) Pins 4 and 5 are the grounds. A simple test light connected across them will tell you if there is power there. However, if you have a typical code reader that doesn't have a battery in it you can tell if there is power as soon as you plug it in. If it comes on there is power. However, there are 2 grounds in that system. One is inside the vehicle in the right side footwell behind the trim. The other connects to the engine at the top rear. It's possible the second one is not connected or more commonly it's corroded. The attached may help as well. The 2 green are the ground and the red is the battery power.

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Saturday, May 17th, 2025 AT 12:26 AM

BLAZ85

MEMBER
54 POSTS

First, thank you for your quick response.

Now an update: As usual, you nailed it. It was the ground in the engine compartment. It was possibly disturbed during the manifold replacement but was also corroded. We cleaned it well and tightened well. Plugged in code reader and it connected displaying a P0305 code (Cylinder 5 misfire). Further inspection showed a small coolant leak at the thermostat housing which allowed coolant to leak onto Number 5 coil. We repaired the small leak, installed a new coil and cleared the code. The vehicle now performs flawlessly.

Thank you so much for your help! You guys are the greatest! Have a great day!

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Wednesday, May 21st, 2025 AT 10:11 PM

STEVE W.