Piston Slap: Sounding the Alarm on a Bad Sensor?

Sajeev Mehta
by Sajeev Mehta
piston slap sounding the alarm on a bad sensor

Diana writes:

Hi Sajeev!

My husband, rrhyne56, gave me your email address, because I have a question about my 2008 Honda CR-V.

The alarm goes off, in the middle of the night, only when the temperatures are freezing. Unlocking with the FOB, stops it. But, the FOB will not lock the car. I have to open the door, close it and then the FOB will lock and set the alarm again.

Hope you can help, because I’m losing sleep. Maybe my car wants us to move to a warmer place… 😉

Sajeev answers:

I don’t work on commission, yet I love getting referrals!

Assuming this is a factory Honda alarm, I’ll bet there’s a bad door/hood/hatch sensor freaking out the system. The Internet says a weak battery is a major concern, so ya better test that first.

If the battery passes but there was recent collision repair, I reckon the affected area also needs a new sensor. More to the point, after I wrecked my Mark VIII, one of the first things I did was disconnect the hood sensor: it mercilessly honked as I tore off damaged panels. Since my diagnosis was pretty damn obvious, I bought an NOS Ford sensor and cleared one of the easier hurdles on that project.

If the CR-V has no recent collision repair, either disconnect or clean the sensors. I couldn’t google the Honda diagnostic, but the logic is to disconnect a sensor, tricking the computer into thinking everything is fine. The trick might also entail jumping the wiring with a metal paperclip. Watch this video, ask someone smarter than me and/or experiment yourself by starting without the paper clip.

I reckon you need a new hood sensor, as it had enough of having a hood slam down on it. I certainly don’t want that thing slamming down on me for 12 years.

What say you, Best and Brightest?

[Image: Honda]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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3 of 35 comments
  • Nlinesk8s Nlinesk8s on Feb 01, 2020

    I this was a VW, it would definitely be the door-sensor microswitch; they wear out with these symptoms.

  • Poltergeist Poltergeist on Feb 01, 2020

    The factory alarm is not activated by the door pin switches, it is activated by the internal switches in the door latches. If the body module senses a door unlock, without the key fob or key being used. it sounds the alarm. Most likely on this vehicle, one of the door lock actuators (which are part of the door latch assy) is seizing, which causes the internal switch in the latch to trigger. Easiest way to check is to manually lock/unlock each door with the inside knob. If you feel one or more difficult to lock/unlock, replace that latch assy. There is a safety recall for the LF latch/actuator, so if it's that door, it may be covered by Honda.

    • ToolGuy ToolGuy on Feb 01, 2020

      This makes sense - because if I break into your car, I'm going to *unlock* the door before I open the door (whether I break the window or use a lockout tool or use a wedge to reach the lock switch). Diana, how old is the battery? A relatively weak battery which gets even weaker in colder temperatures would be the simplest explanation tying the alarm to freezing temperatures. As Sajeev suggested, many automotive parts stores will test your battery for free. IF the battery is good, set the alarm (lock the doors with the fob) from outside the vehicle and pull on the hood and then the liftgate (the same way that ice/snow might push up/out on them as it gets colder and the ice expands). Does that set the alarm off? (If it is the rear liftgate, check for dirt/debris in the rear liftgate seal, clear it out and try again.) Now try the same thing with each of the doors (from outside the vehicle). Then, sitting *inside* the car, set the alarm by locking the doors with the fob, and then try poltergeist's tip of manually unlocking each door and setting off the alarm [we think] - does one of them feel different from the others or set the alarm off more 'quickly/easily'? Interestingly, the front right (passenger side) and rear right Door Lock Actuators are some of the most popular parts sold on rockauto.com (denoted by the little heart symbol) for the 2008 CR-V: https://www.rockauto.com/en/catalog/honda,2008,cr-v,2.4l+l4,1441752,body,door+lock+actuator,13257 [If it turns out to be one of the door lock actuators and you do the repair yourself, be careful doing it in the cold - the plastic door panel parts you need to remove will be more brittle in cold weather.]

  • Jeff71960 once a fun fast little car (if you can find an unmolested one)... unfortunately boy racer types trashed most of themhttps://www.cargurus.com/Cars/l-Used-Dodge-Neon-SRT-4-d658
  • Pig_Iron How many second chances does Farley get? Is there a plan to deliberately destroy Ford? 😞
  • Tassos Neons, new, used, or junk like this one, were the right car to own if you wanted it advertised what a lame loser you were.
  • Damage My mother had a 78 with the FI motor. If you wound it out in first (not that she ever did) it would reward you with just a little tickle of torque steer. It was pretty reliable until water leaks from below the windshield found the fuse block. Once that was fixed, it was good for several more years. Eventually it got rusty and was sideswiped by a snowplow, and she sold it to my coworker who got several more years out of it. She traded it for a Mk2 Jetta, which was a fun little car. I don't miss the Rabbit but I'd love to find a clean Jetta again.
  • Tassos in the same league as Tim's so-called "used deathtrap of the day" today.Both emiently junkworthy,