So I have a 2000 Honda Civic HX I just recently did a head gasket water pump and timing belts on it and after I did all of that, I noticed that the VTEC oil pressure switch. I accidentally ripped the wires out of the connector so I ordered a new connector that had wires already on. It made sure that I soldered the wires correctly And I still have the P 1259 I even tried replacing the entire VTEC solenoid and the pressure switch and it’s still not engaging. I went home and did a resistance test on the old oil pressure sensor in the new oil pressure sensor with no pressure. They read zero ohms and they read zeros even with pressure as well. I read somewhere that it’s supposed to read an open loop when it’s open and zero ohms when it’s closed I did a resistance check on the connector and I have continuity. I did a vault check on the connector. I had 11.8 V going to it with Keon engine off. I have not checked engine on. what I’m gonna do is pull the solenoid off of the base and see if the plunger is stuck on the new one I checked voltage going to the solenoid and I have 12 V there I’m not able to rev the car past 4000 to where VTEC would engage because it’s in limp mode and stuck at 4000. I’ve tried a different ECU. The only thing I haven’t done is check if there’s continuity between the ECU and the oil pressure switch and I haven’t checked if the oil pressure switch changes if I give the car gas and I haven’t checked if there’s a voltage drop when I rev the car

 

It sounds like you're dealing with a persistent P1259 code (VTEC system malfunction) on your 2000 Honda Civic HX, despite replacing the VTEC solenoid, oil pressure switch, and connector. Let’s break this down systematically based on the information you’ve provided and suggest next steps to diagnose and resolve the issue.

Recap of the Issue

• Symptoms: P1259 code, VTEC not engaging, car in limp mode (limited to 4000 RPM).
• Work Done:
  • Replaced head gasket, water pump, and timing belt.
  • Replaced VTEC solenoid and oil pressure switch.
  • Repaired VTEC oil pressure switch connector (soldered correctly).
  • Tested resistance on old and new oil pressure switches (both read 0 ohms with and without pressure, which is unexpected).
  • Confirmed 11.8V at the oil pressure switch connector with key on, engine off (KOEO).
  • Confirmed 12V at the VTEC solenoid.
  • Tried a different ECU, but the issue persists.
• Pending Checks:
  • Continuity between ECU and oil pressure switch.
  • Oil pressure switch behavior under engine operation (with RPM increase).
  • Voltage drop at the connector when revving.
  • Inspecting the VTEC solenoid plunger for sticking.

Understanding the VTEC Oil Pressure Switch

The VTEC oil pressure switch in your Civic HX (D16Y5 engine, VTEC-E) is a normally open switch. It should:

• Read open circuit (infinite resistance) when there’s no oil pressure (engine off or low RPM).
• Close (0 ohms) when sufficient oil pressure is present (typically above 3000–4000 RPM when VTEC engages). Your finding that both the old and new switches read 0 ohms with and without pressure suggests either:

1. Both switches are faulty (unlikely, especially with a new one).
2. The testing method is incorrect (e.g., not applying proper oil pressure during testing).
3. There’s an issue with oil pressure reaching the switch.

Diagnostic Steps

Here’s a step-by-step plan to pinpoint the issue:

1. Verify Oil Pressure at the VTEC Solenoid

The P1259 code can be triggered by insufficient oil pressure, which prevents the VTEC system from engaging. Since you recently did a head gasket replacement, ensure the oil system is functioning correctly.

• Action:
  • Remove the VTEC solenoid and inspect the plunger and screen for blockages or sticking. The plunger should move freely, and the screen should be clean.
  • Use an oil pressure gauge to test actual oil pressure at the VTEC solenoid port (engine running, ideally at 3000–4000 RPM). For the D16Y5, VTEC engagement typically requires ~50–60 psi of oil pressure.
• Expected Result: If oil pressure is low, check for:
  • Low engine oil level or incorrect oil viscosity (use 5W-30 or as specified).
  • Clogged oil passages in the head (common after major engine work if debris is present).
  • Oil pump issues (though less likely if the engine runs fine otherwise).

2. Test VTEC Oil Pressure Switch Operation

Your resistance test showing 0 ohms regardless of pressure is concerning. Let’s verify the switch behavior under actual engine conditions.

• Action:
  • Disconnect the VTEC oil pressure switch connector.
  • Connect a multimeter (set to continuity or resistance) across the switch terminals.
  • Start the engine and observe the resistance/continuity:
    • At idle: Should be open (infinite resistance/no continuity).
    • At 3000–4000 RPM (or as high as limp mode allows): Should close (0 ohms/continuity) if oil pressure is sufficient.
• Expected Result:
  • If the switch remains closed (0 ohms) at all times, it’s likely faulty or there’s a short in the wiring.
  • If it remains open, there may be insufficient oil pressure or a faulty switch.

3. Check Wiring Between ECU and Oil Pressure Switch

Since you haven’t confirmed continuity between the ECU and the oil pressure switch, this is critical. A broken wire or poor connection could cause the ECU to misread the switch state.

• Action:
  • Locate the VTEC oil pressure switch wire (typically a single wire, often green/yellow or similar, going to ECU pin D6 on the D16Y5).
  • With the key off, test continuity between the switch connector and the corresponding ECU pin (consult a wiring diagram for your Civic HX, as pinouts vary).
  • Check the ground side of the switch (usually grounded to the engine block or head).
• Expected Result:
  • Continuity should exist between the switch and ECU.
  • If there’s no continuity, trace and repair the wire.
  • If there’s continuity but the wire is shorted to ground or another circuit, this could explain the P1259 code.

4. Verify Voltage at the Switch with Engine Running

You measured 11.8V at the switch connector with KOEO, which is reasonable (slightly low due to battery voltage). Now, check the voltage with the engine running.

• Action:
  • With the engine running, measure voltage at the switch connector (between the signal wire and ground).
  • Rev the engine to ~3500–4000 RPM (or as high as limp mode allows).
• Expected Result:
  • Voltage should drop to near 0V when the switch closes (due to sufficient oil pressure grounding the circuit).
  • If voltage remains constant (~12V), the switch isn’t closing, indicating either a faulty switch or insufficient oil pressure.
  • If there’s no voltage, check the ECU’s output or wiring.

5. Inspect the VTEC Solenoid Operation

Since you’ve confirmed 12V at the solenoid, verify that the solenoid itself is functioning.

• Action:
  • Remove the solenoid and apply 12V directly to its terminals (use a fused power source to avoid damage).
  • Listen for a click or feel the plunger move.
  • Ensure the solenoid valve is clean and not clogged with debris from recent engine work.
• Expected Result:
  • The solenoid should activate. If it doesn’t, the new solenoid may be defective (rare but possible).
  • If it activates but VTEC still doesn’t engage, oil pressure or ECU signaling is likely the issue.

6. Clear and Recheck Codes

After each test, clear the P1259 code using an OBD-II scanner and see if it returns. This helps confirm whether your repairs are addressing the root cause.

7. Additional Notes

• Limp Mode: The 4000 RPM limit is typical when the ECU detects a VTEC fault. Resolving the P1259 should restore normal operation.
• ECU Swap: Since you tried another ECU and the issue persists, the ECU is unlikely to be the problem, assuming the replacement was known good.
• Post-Repair Debris: Head gasket replacement can introduce debris into oil passages. Double-check that the oil system is clean, as a clogged passage could prevent sufficient pressure from reaching the VTEC system.
• Grounds: Ensure all engine grounds are secure, as poor grounding can cause erratic electrical behavior.

Likely Causes (Based on Current Info)

1. Insufficient Oil Pressure: Low pressure at the VTEC solenoid due to a clogged passage, oil pump issue, or improper assembly during the head gasket job.
2. Faulty Oil Pressure Switch: Both switches reading 0 ohms is suspicious. Ensure your test method is correct (e.g., not shorting the terminals during testing). Consider sourcing another switch from a reputable supplier.
3. Wiring Issue: A short or open circuit between the switch and ECU could cause the ECU to misinterpret the switch state.
4. VTEC Solenoid Issue: Even a new solenoid could be stuck or clogged, preventing proper operation.

Next Steps

Start with:

1. Checking oil pressure at the VTEC solenoid port with a gauge.
2. Verifying the oil pressure switch behavior with the engine running.
3. Confirming continuity between the switch and ECU.

These tests will narrow down whether the issue is mechanical (oil pressure), electrical (wiring/switch), or component-related (solenoid/switch).

(Grok AI via x.com)

 

Did this ever get resolved?

I'm having the same issue after doing a head gasket and I see in here that there is no mention of the seal between the camshaft housing and the head itself. If there is enough oil pressure leaking out of the seal that is supposed to be created with oil-resistant RTV between the head and the camshaft housing, then the oil pressure switch will correctly read that the oil pressure is low inside the VTEC solenoid.

I'm thinking that I will need to pull the camshaft back out and re-seal the housing to the head.

There is also something about an oil pressure air plug in the camshaft... I need to re-read the section of the Alldata section.

 

My issue ended up being a bolt loosening and coming out that plugs the VTEC diagnostic port. It's basically a big M10 plug that's only ever used for diagnostic testing.
Same one as this post - https://honda-tech.com/forums/honda-...rking-3371828/