Honda P2646 Code: Why Your VTEC System Isn’t Working

Most Common Causes of P2646 on Honda Engines

Quick Summary
A P2646 trouble code on a Honda almost always traces back to the VTEC oil pressure control system. The Honda P2646 code means the engine computer isn’t seeing the correct oil pressure signal from the rocker arm oil pressure switch when the VTEC system activates. In simple terms, the engine can’t properly engage the variable valve timing system. The most common causes of P2646 include low engine oil, dirty or sludged oil, a clogged VTEC screen filter, a failing VTEC solenoid, or a bad rocker arm oil pressure switch. The good news is that many Honda P2646 repairs are relatively simple, often involving cleaning the VTEC screens, replacing the solenoid assembly, or correcting an oil maintenance issue.

P2646/P2651 (rocker arm oil pressure switch circuit low voltage).
P2647/P2652 (rocker arm oil pressure switch circuit high voltage).

Article

P2646 Honda Code Explained: Causes, Diagnosis, and Fixes

Over the years, I’ve diagnosed quite a few Honda engines that came into the shop with a P2646 trouble code. In most cases, the customer complaint is the same: the vehicle feels sluggish, the check engine light is on, and the engine doesn’t seem to pull like it used to.

When I see a P2646 code on a Honda, I immediately think of one system—Honda’s VTEC system. This variable valve timing system relies heavily on proper oil pressure and clean oil passages. If something interferes with that oil flow, the engine computer detects it and sets the P2646 code.

Understanding what P2646 means and how the VTEC system works makes diagnosing the problem much easier.

On a Honda engine equipped with VTEC, the computer controls a solenoid valve that directs oil pressure to the rocker arm assemblies. When that oil pressure builds, it locks the rocker arms together, allowing the engine to switch to a more aggressive cam profile for better power at higher RPM.

When the system activates, the ECU expects to see confirmation from the rocker arm oil pressure switch. If the ECU commands the system on but the pressure switch doesn’t report the expected oil pressure, the computer sets the Honda P2646 code.

In other words, the engine computer is saying: “I turned on the VTEC system, but I didn’t see the oil pressure signal I expected.”

When that happens, the engine may disable VTEC operation entirely.

How to fix P2646 Honda trouble code

Most Common Causes of P2646 on Honda Engines

• Low engine oil or dirty engine oil — Often, the first fix for Honda P2646 is simply performing an oil change with the correct viscosity oil.

• Clogged screens due to dirty oil — There are two screens used in the Honda VTEC system; one is at the VTEC pressure switch, and the second one is on the front of the cylinder head behind the power steering pump. These screens get blocked by sludge

Honda VTEC solenoid spool valve

from dirty oil, or a malfunctioning pressure switch, triggering a “stuck off” code and restricting engine RPM

The VTEC solenoid spool valve has a small

metal mesh screen gasket (located on the cylinder head) that filters oil. If this screen gets clogged, oil pressure cannot activate the VTEC system properly.

Locate the VTEC Solenoid: On the back of the engine, near the power steering pump.
Clean/Replace Filter: Remove the solenoid assembly, clean or replace the filter screen and the gasket.

Honda issues a service bulletin for an updated oil pressure switch

Honda has issued a service bulletin #13-021 to address the problem on the vehicles listed below.

Models affected by P2646 Honda and service bulletin #13-021

2003–12 Accord L4
2012–13 Civic ALL except Si and Hybrid ALL
2002–05 Civic Si
2002–09 CR-V
2011 CR-Z
2003–11 Element
2007–11 Fit

Based on the service bulletin, the rocker arm pressure switch can fail on an intermittent basis. Honda has issued an updated part; pressure switch 37250-PNE-G01 and O-ring 91319-PAA-A01.

How to test the VTEC oil switch

The VTEC oil switch is a normally closed style switch. The ECM provides a reference voltage to the switch on the blue/black wire and when the switch is closed, the reference voltage is grounded, so the ECM expects to see a large voltage drop. The ground wire is brown/yellow. Start your diagnosis by checking for good ground on the brown/yellow wire. Next, backprobe the connector to the oil switch and check for reference voltage on the blue/black wire with the engine running.

What’s going on with the bad VTEC oil pressure switches

The defective switches are going into the open state at RPS 2500 and less when they should be in the closed position.

Other causes for  P2646 Honda

If you’ve replaced the VTEC oil pressure switch and still have code P2646 at RPMs in the 2500-400 range, you’ve got a low oil pressure issue, dirty oil, clogged VTEC screen or a problem with the VTEC assembly. In that case, make sure the oil filter is new to ensure the pressure isn’t restricted or in bypass mode.

©, 2020 Rick Muscoplat